I don't valet if I don't have cash on me, so I rarely ever valet. Only places I have were at J. Alexander's ("free") and the Hilton in Daytona Beach (mandatory for their garage). Generally I do $5 in/$5 out.
I always wondered whether you should tip on the way in or the way out. I don't think I've ever valet parked my car being out in the suburbs, but a five-ski in and out seems fair to me. Now I want to find somewhere and do it just for the experience.