We recommend a $1-2 tip is quite acceptable for hailing a cab, however if they are actually picking up a car for you $2-5 is appropriate.  If you're in a hurry you can tell them that one of you needs a washroom or feels ill, they may speed it up a bit with a larger tip.  Sometimes valet in Vegas can take 30 minutes before you get your car.

NOTE: Most hotels have free parking.  Check with your hotel prior to arrival.