Bill,
I'm sure I replied to this yesterday, but must've forgot to hit "submit" (duh)
My thoughts are these:
1) Most arranger keyboards are already full of compressed sounds and styles. Unless you're concerned with live input (like vocals, or guitar) I can't imagine really needing to add compression to a keyboard mix. They all sound like records ready for airplay to me.
2) Compressors are sometimes more trouble than they are worth. There's a fine line between sweetening a sound, and stripping it of it's dynamic integrity

There are other setting specific thoughts I have, but we'd need to discuss these as they relate to your situation.
With THAT said, why do you think you need to add compression? Is there a sound, or style that isn't sitting (setting?) well in the mix for you? Some other reason?

I rarely use it in my live rigs. The studio is another animal, but that's a story for another day.