Just like Tim said
"By myself for as much of the process as I can get away with. I only like the client in the room for a final tweak or two... doesn't allways happen that way but that's my preference."
However I do need them back:
To show off to
To make me feel bad that I missed something
To get their input
Because I get stuck after a while
Mostly because I like to do fader moves with an audience
I feel I hav been "off to Mars" while on my own, having the clients there "brings me back"