If I want to run a suppressor on my piston AR, do I need an adjustable gas block?

Which GP system do you have?

I have a straight-out-of-the-box LWRC M6 in 5.56 and have no experience with suppressors whatsoever but would like to get into it as soon as my budget supports it.

If the problem with suppressors & DI ARs is that you have to dial back the pressure when running a suppressor to keep from getting excess gas in the face, it seems like a piston would prevent most of that from happening (because the excitement is all happening at the front of the piston), and therefore not need reduced pressure...

Am I wrong?

Run your LWRC in the suppressor setting of your piston...you have 3 settings already.  Full suppressed and off at least both of my LWRC M6's have that

not all lwrc piston guns have an adjustable gas block

you definitely want one for suppressed. LMT and Caracal here and they are both awesome suppressed, but wouldn't be if they didn't have adjustable gas blocks.

No, you do not NEED one.

It's nice to have because it reduces the gas blow-back and possibly (depending on gun, can, ammo, etc.) can help in reducing recoil from an over-gassed gun and maybe (big maybe) wear on the bolt. Functionally though, there isn't a need.

This is correct. The thing you might want to consider when running a can on your LWRC if it is the no adjustable style would be a heavier buffer to slow down bolt speeds and save your extractor.