I use the transpose button down 1 whole step most of the time. This puts many songs in my voice range and allows the tenor sax and trumpet to read off the same chart. Most Beatle tunes I have to go down 4 half steps. Sometimes I sing a song an octave down and use the transpose button to bring it up. An example of this would be "Georgia On My Mind". I play it in the key of "C" and transpose it up 1 half step.

Don is correct in that if you use the button to transpose it too far, it will sound unnatural.