Recently my LGS asked a player to stop coming because he was 3d printing HUGE amounts of his army. As in, 3d printing most of a knight, more than 50% of a marine, etc. That seems like a fair ask really. At some point the model becomes a proxy, and is not a valid model. That said, no one has ever balked at kit bashing a model, but this tends to be a slippery slope. The rule our LGS went with was 70% legit pieces, or special conversion reasoning. Replacing shoulder pads, and such no one cared about. The torso & legs, I could see an issue there for an LGS. You could always ask (or beg forgiveness later, but it would be too late if you already assembled everything). For the record, 2nd hand marines are CHEAP (got over 2300 points worth of troops, vehicles, everything for like $100)! You should be able to get what's needed without 3d printing 'most' of marines. Other armies not as much.
While personally, i don't care. It's about where you're playing in my opinion.
At Ordo, no one at the clubhouse is going to care in a friendly game (speaking for myself anyway). If at a tournament, I suppose it would depend on proxy rules.