From the Youtube guide I'm using... It makes it sound like you always should check headspacing, which of course, requires more things to buy to test it... As part of my two builds, I bought two complete uppers from Palmetto State Armory - It was my understanding that you don't need to check headspacing when you start with a complete upper... Is that correct?

its already headspaced at the factory

It never hurts to check it, but with a complete factory upper and BCG, it's also usually not necessary.

I am not an expert and have factory builds and my own builds.  I do check headspacing- just because, but have never found anything even close to being out of whack.  However, all of my 556 chambers measure as 223 except one, and that is a BCM BFH barrel that seems to be reamed properly, albeit a bit tight.  So chamber measuring/reaming is far more important to me.
I use the 223/556? gage:  http://m-guns.com/tools.php 

I think my set cost $40 when I bought them, I check

“Down and Dirty” Headspace Gauges
here

Unless you're going to build many rifles, would it be cheaper to just go to your local gun shop and let the gunsmith check it for you?  They might even do it for free if you're buying something or using the range.

I always check headspace.
I don’t buy from PSA, but I have seen ‘factory’ built uppers fail headspace with the bolt that shipped with the upper.

Even though I have never had a problem with headspace, I really do not trust the inexpensive commercial assemblers of today.  I think the lower end ones just throw together the least expensive parts they can find and ship them quickly.  Example:  I was in a rush and purchased an inexpensive upper from CBC Industries earlier this year for use as a truck gun so I did not want to invest much money into it.  One of the reasons I purchased from CBC was they stated they tested each before shipping.  I had a much more expensive 300 BO on order from RRA but it was on a long-term backorder.

I cleaned the CBC upper, installed the sights, etc and went to the range to zero.  I had every type of feeding and ejecting problem one can imagine.  Plus material was being shaved off the bullets as they were fed and the shoulder of the cases were being dented.  The only way I could fire this was to manually load each cartridge into the chamber.  Long story short, the barrel extension appeared to be rotated a couple degrees when installed which screwed up the feed ramp geometry, along with the gas block just sliding down the end of the barrel once I got it home for inspection. 

So as is typical for me- trust but verify anything important.  I even checked the headspace on my new BCM upper- and found it to be just fine, as expected.

Complete PSA upper? Most are shipped with no BCG or CH. I check all of mine, including my Sig M400 that is my only factory built AR. It's cheap insurance.

In my opinion, if you are spending money and tinkering with AR builds just go ahead and buy the go/no go gauges. A set costs about $50 and that's a heck of a lot cheaper than a trip to the ER.
You will have them forever and the peace of mind that goes with it. Plus you can charge your buddies to use them

It can only be headspaced to a particular bolt.  If OP did not get the factory bolt it is not headspaced at the factory for him.