I know my highest and lowest comfortable notes, and the high and low limits of my range (notes I can count on, even if I'd rather not sing that high or low), and when I learn a new song, I check the highest and lowest notes in it, then change keys if necessary to keep it as close as possible to my comfortable range.

The main thing with an accompanying instrument such as the guitar is to accompany the song in whatever key is best for your voice--not what's easiest to play on the guitar. Use a capo when necessary.

You are right, Don, and that is fine if you are singing solo either self accompanied or accompanied by someone else on a guitar or other fully chromatic instrument.

On the other hand, if you are being accompanied by a diatonic instrument (not uncommon in Britain) or are singing with others with a different range to your own then some compromise on choice of key may well be necessary.