First of all, heavy lifting should always be rewarded. But you knew that. They're management, and they're in training and, in fact, should refuse money if you offer it.
Most hotels add a service charge to the bill.
Some guests don't notice that, and I imagine many a room service waiter has skipped back down the hall with a 30 per cent tip. That way, a server knows you mean to acknowledge his or her effort. If someone has gone beyond the call of duty, or if the built-in gratuity is only 10 or 12 percent, you might want to add a few bucks on the top. Better hotels offer a concierge desk to help you with restaurant or theater reservations. Any extraordinary effort by a concierge, such as securing sold-out tickets, deserves a ten or twenty spot.