I try to tip both sides of the valet equation but if I am lacking cash on the way in, the guy on my way out is going to get double. In my mind they are sharing, whether it is true or not. If I don't have cash on both ends, I'm putting a tip on my room. If the hotel charges for valet, I may be tipping less than if they are not.
Tip story from that job that sys will enjoy: An Event Manager was putting on a wedding and the couple was doing a brunch the next day. They wanted an OJ station and I believe the cost for OJ was something like $20 a gallon. They didn't want to pay that (weird line to draw in the sand when you are having a $35k wedding) and complained a bunch to the manager. The manager eventually put the price at $15 in an effort to accommodate them. Their wedding weekend, they told her they had a tip for her. Then in front of their family/friends they gave her a gallon of OJ.