You don't have to true receiver faces... But, I have had precision improvements from all the AR's I have done it to.
The choice is yours. Clearly all sorts of AR's shoot great groups without it being done.
That said....I have seen consistent precision improvements , on my Large Frame's and AR15's to merit the added effort.
Lots of people say buy quality parts and you don't need to... every upper I have lapped showed an uneven face... ironically the one that showed the least amount of unevenness was a PSA AR15 upper. ( Not sayin' the PSA's are perfect )
BCM, Rainer Arms, SOLGW, SanTan , and more all had high spots the lapping tools help level... can explain why the CNC machines variances.. no but I have chalked it up to un even Anodizing thickness... can I prove it ? ....Nope....
IMHO, the lapping won't hurt anything , provided you use a well fitting quality made lapping tool.
Here is an example of my results with a OEM PSA / DC Machine barreled 6.5CM... ymmv, and this is what I based my decisions about lapping on.
My Criterion barrels and FN Barrels all showed even better , more consistent improvements with factory ammo.
Can I "prove" the lapping was "the" specific improvement ? Nah... but it sure seemed to consistently help with better precision.
Every upper I have done it too.. has had improved precision. Was my testing "Scientific"... nope.. it was just seat of the pants.
Ultimately , to each his own... so do as you want.
It is so easy to do, I can't help but do it to all my uppers... I have yet to see any noticeable degradation in precision from any lapping and Loctiting ( True and Glue , TnG )
Does anyone need to do it ? Nah... but for me , why not ?
https://www.ar15.com/forums/industr...-again-UPDATED-with-Tn-G-results-/301-285762/