I generally have my transpose on most of the time. I've found that many of the voices on my keyboard (a now archaic Ketron MS60) sound richer and more mellow when transposed down a couple of half tones or more.

When at times I play with a trio, the guy who usually books those gigs and who plays trumpet with us has us all use "C Concert" fake books. So I make my keyboard a B flat instrument, like the trumpet and the sax, so we all read off he same music and all play in the same key. (The horn players don't use B flat books in this case). The tenor sax player doesn't like that too much, because it is not the best sounding range for the sax, but the trumpet guy finds he doesn't have to hit those real high notes, which are a challenge for him.

When I'm playing solo and doing some vocals, I find most of the tunes are written in keys that are a bit too high for my voice. I've found that transposing down 4 half tones suits my vocal range much better, so that's what I generally do.
Paul C.