I pretty much always tip. If someone touches a bag, I'll give a dollar a bag when they put them on the cart if they're going to store them (kind of an insurance policy in my mind, not that they'd likely treat them any differently had I not tipped). Once they deliver them to my room, I'll normally give $2 per bag, maybe $3 if it's raining or they have to haul them up stairs. Don't know how that compares to what you're supposed to tip, but when you really think about it, spending thousands of dollars on a trip (between airfare, park tickets, transfers, annual dues, etc.) what's a few bucks here and there?