My rule of thumb when playing solo is to pitch a song in whichever key ensures that the highest note in a song is the highest note I can sing comfortably. In my case this would normally be an "E", although sometimes I will take it down so that the highest note is a "D".

But that is just what suits me. My experience is that women tend to sing naturally about a fifth above the male voice, so a song which I would sing in, say, in the key of D, will sound better transposed into the key of A if I am accompanying a woman.

Not to drift too far off the thread here, but in what key do you sing "Happy Birthday" ? Normally I sing it in G, but my guess is that most women would prefer to sing it in D. But for a male to sing it in that key he would have to be either a basso profundo or a castrato. (Or an expert harmonizer).

Murray