Just got my FT-891 and am taking my General exam in the next few days (currently tech). I currently have a 80m simple dipole NVIS setup that I've used with a tiny 1/2 watt QRP CW rig with some success, but that's pretty much the extent of my exposure to HF anything.

First question: do I need an antenna tuner? I have a ton of (wooded) space around my house (tens of acres) and there's no such thing as an HOA out here. My thought is to set up a couple more dipoles for 40m and 10m, and maybe get a dedicated vertical for 6m ssb to talk to some friends (terrain sucks for UHF simplex). I have a VNA and can probably tune each antenna when I first set them up. The unit that integrates directly with the 891 is decently pricey, and if it's not really necessary, I don't mind skipping it.

I'm not planning on trying to do any crazy DX work etc., just want to send and receive signals regionally and play with digital.

You don't have to have a tuner.  The problem though is 80 meters - the band is large enough that it's very difficult to get one simple antenna to cover the whole band without a tuner. There are some tricks people do to increase bandwidth, if you have plenty of space you can even do a fan dipole with widely spaced elements of different lengths to get the top and bottom of the band. Or you could set up a vertical tuned to the bottom of the band for DX and a dipole for the top of the band for voice or something like that.

The higher frequency bands are able to be covered adequately with simple antenna designs without a tuner.

If your friends are using verticals on 6m also that will be fine, the convention is horizontal for 6m SSB but either will work fine as long as everyone you want to talk to follows along.

As Gamma says, you don't need it.  Back in the day,  people (my father, for example) would simply cut a wire antenna to the calculated length, string it up, and work the world. But I do think a tuner will give you some versatility.  For instance, I use a resonant 20m dipole in my attic.  But the tuner in my rig (Elecraft...they make really good tuners) will allow me to use that same antenna on 12, 15, 17, and even 30 meters.  So that is 5 bands with one wire antenna.  And while it would be nice to have a  resonant antenna for each band, that would mean a lot more work and maintenance.  

FWIW, YMMV, etc.  

A good tuner is a nice tool to have, but not always needed.  It depends largely on what type or types of antenna you are using and how agile you want to be across a band and between bands.

First thing to note, the term "tuner" is a bit of a misnomer, and a lot of newer hams interpret it to mean it brings your antenna into "tune" to make it work better.  In reality. all the tuner is doing is correcting an impedance mismatch between the antenna feedline and the radio's coax port.  Your radio expects to see a 50 Ohm impedance at the coax connection, and if that is off from 50 Ohms, you get incomplete power transfer and some of the energy that should be sent as signal to the antenna gets reflected to the radio circuitry and bled off as heat.  Do that too much and something in the radio will break, so the tuner is there to protect the radio and ensure a better transfer of power to the antenna.

A tuner can make your life easier, you can have an antenna that's not itself resonant on all the bands you want to work, but the tuner can at least correct the impedance match enough to allow you to use it, even if the antenna isn't exactly resonant there itself, it will still put RF into the air.  That means needing less "hardware" in the antenna farm to work more bandwidth.

Or, if you rather, you can get antennas that are themselves resonant on all the bands you want to work and feed them directly with your radio.  This usually means more antennas, or using antennas that have discrete components built into their designs that facilitate muliti-band use.  This can be a bit more complicated and equipment-intensive, so it just depends how you want to go.

And Gamma made a good point, on some bands like 80M where the whole band is very wide, even a resonant antenna for that band might not give you a good low SWR across the whole thing, and there having a tuner would be nice to "patch up" the match on the far ends of the band.

Interesting.    I know so little but have had some exposure in the military.   Some of the older vehicle radios utilized a “matching unit” at the base of the antenna.   I want to I remember if you switched the receiver, the matching unit would buzz mechanically a couple seconds.  

Just musing that your post brings some things together.

Used to set up a fair amount of 292 antennas usually for a PRC 77.   90% of the joes would screw on every copper rod section, never knowing to “cut” the antenna to the freq used or to set it up for directional broadcasting.......despite the manual being in the 292 kit carry bag.   Manual also had improvised antennas directions with comm wire and MRE plastic spoon insulators.

I wish I had a surplus 292 kit to set up on the lawn.   They were easy enough to set up.

The MRC-138 comes to mind.  It was a PRC-104 on steroids.  It had what they called a coupler between the power amplifier unit and the antenna, yes it tuned the antenna for the frequency it was going to use.  Every time you changed freq and then keyed the handset the power amplifier would reset and send the coupler to a "home" position.  Then it would power the unit and tune for roughly 10-25 seconds depending on the freq selected.  You could hear the motor driven tuning process happen during this time.  The power amp could be selected for 100 or 400 W depending on what you needed.

Basically the same concept if I am not mistaken is happening here, just not military gear built by the lowest bidder in this case.  Your wallet will determine how effective it is...

Another option is the ladder line fed doublet. Cut it for the lowest frequency band you want to operate and use an antenna tuner with a built-in balun for the higher frequency bands. Ideally, the ladder line will run all the way back to the shack, but the balun can be placed anywhere in the feedline.

The SWR will vary widely depending on the frequency, but the loss of ladder line is low, thus so is signal attenuation.

You could place the balun just outside the shack and run coax for the short remaining distance to the tuner, thus reducing losses.

Doublets are also easy and relatively inexpensive to make. You can even make your own ladder line. Antenna components are available here.

The ARRL Antenna Book is a worthwhile addition to any ham shack. It changes very little from year to year, so it's a long term investment.