Fair point! To an extent you have to when playing with a keyboard or similar, but I expect it's a bit of a compromise between systems even then. I'm not classically trained, though, and couldn't even tell you for sure what I do, though I know when it sounds right and when it doesn't; and that seems to vary according to context. The school-trained players do seem to have less of a tendency slightly to sharpen or flatten certain notes, I think, than traditional or "vernacular" players, and the same would likely be true of singers; but since I don't properly understand the processes involved, I certainly wouldn't want to be dogmatic. My original comment may well be misleading given the way I put it.