If you have to transpose in any direction to make it comfortable to sing then do it, and if it works then that is the right key. The first thing to do with ANY song is to find out what key suits YOU. It's your voice and singing in the wrong key is the easiest way to ruin your voice and sound bad at the same time.

Experiment. Some songs that I sing (with my partner in a duo) do not sound right at the "comfortable" key. We sometimes do "Hungry Eyes" (Eric Carmen, from Dirty Dancing). The comfortable key for me would be Bb, giving a top note of F. However, singing the song in this key sounds flat, so we play it in the key of C - top note G - and then it sounds good because of the extra effort required to sing the song.