As ever, it's a broad debate put here in a brief soundbite.
We all have our own tolerances but I think most would agree you shouldnt have to correct defects in new RTR (such as, hypothetically, a loco that wont run properly or a coach that had the wrong roof profile) - but then again if you want to do it, that's OK. Modifying to enhance, to detail, to represent variations or different builds using a basically sound model is a completely different thing, and should indeed be 'what we do'.