Since it is the easiest for me I use the transpose key. For the majority of songs I can transpose to the popular keys mentioned above on the fly. If I play by ear I can usually do most any key for backup using mostly the I, IV and V7 combination to cover most singers.
Recently I was backing up a singer and needed to change tempo (they were singing show tunes) while they held certain notes and sped through others. I was using the styles and playing along but they don't change tempo mid song easily (I'm on a PSR2000).
If I want to accompany someone like that is it best to just set it as piano or strings or something and forget about the drum and base keeping rhythm?

- Brian