I took a pistol class this weekend and discovered an unexpected difference in reload ease based on having bullets "forward" verses back in a vertical pistol mag carrier.  I had never considered this before, or had any formal instruction, either.  This caused me to wonder (I'm new to training/classes other than shotguns), what is the conventional wisdom on having an AR mag carrier (on a belt) set up with bullets either forward or backwards?  Or does this totally depend on someone's reload technique?  I don't want to have to "re-buy" any gear or "unlearn" any bad habits if I start taking a few carbine classes.  Thanks for any input!

Depends on technique and/or position on plate carrier.
Set it up the way it works fastest and most efficiently for you.

On a plate carrier I like bullets forward, or facing the gun. On a belt I like it facing to the rear for a beer can grip. 

This. Practice and find out what you like best.

Coming off the belt, bullets face to the rear. Beer can grip for me

I'm a lefty and carry three mags on the front of my PC. All are oriented bullets down and tips to my left (pointed towards by gun).

I also carry one on my belt. It is oriented bullets down and tips forward. In practice, that seems to be the quickest reload for me.

Same here.

On a plate carrier I want rounds pointing to the weapon, or dominant side (I am right handed, so for me its rounds pointing right).  This is because you'll need those mags to come straight up to get out of the pouch; best done with the support hand index finger on the front of the mag, thumb between the mag and the chest, pinky - ring on the outside of the mag.  Pull straight up until it clears, then insert in the magwell and give it a solid tug.  A can grip would give you more leverage than what I am describing, but it is hard to get a full can grip from a plate carrier/chest rack.

For a magazine pouch on your belt:

Imagine that your belt buckle is 12 on a clock face so that 3 and 9 are where your hands land when you drop them at your sides.

On a belt if your mag is toward the front - from 11-8 for a right handed shooter or 1-4 for a wrong handed shooter I'd want those mags rounds facing rearward so that I could grab it like a can, get a good solid grip and drive it to the magwell.
If for some reason you have a rifle mag pouch further back than that, anywhere between 4-8, it might be easier to have rounds facing forward and manipulate it like a pistol mag.  Again, like the chest rack, you are not getting as much purchase on your new mag as you would with a can grip.  But if might be worth sacrificing some of that grip so that you have added flexibility on where you are keeping mags.

Some people will say have all your rounds facing the same direction for muscle memory, that's fine, and if it works, cool.  In the end do what works, and look cool.

There are also mag pouches that will carry a magazine either forward or backward.  Such as the Esstac KYWI sold on T-Rex Arms or the Disruptive Environments Carrier at Haley Strategic

On a belt

RIfle reloads face backwards for a beercan grip

Pistol reloads face "Forward"

On a chest rig or PC both face forward due to how I grab them.