i guess this might seem really subjective, but let's say you have an instrumental part that is much easier to play a half step higher. is it worth transposing the whole tune a half step up if it's more playable, even if it sounds better in the original key? and have you ever changed the key of an entire song up or down after the song was written?
well, i use keyboards to record, so theoretically i wouldn't have to actually transpose the sound of the song. but even so, i have no problem playing things in the real key usually. only exception might be if i need a gliss in a different key. to answer your second question, before recording i always do key checks to see what key is comfortable vocally. so many songs are recorded in different keys than they are written