I don't regularly tip unless they go out of their way to do something, help with bags, etc. Then usually $2-$5 depending on how much they helped, how much I've won that day, or even what denomination of bills I have on me.

If all they're doing is waving up a cab and pointing me to it, I don't see much of a point.

FWIW, I used to be a bellman/doorman/valet at a luxury hotel and never expected a tip for just waving a car up out of the taxi pool.