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The smallest bill denomination that you care about is the $20, and I'd suggest getting used to $50's -- they are the new $20's; $100's are just a little over-doing it and the idea of a good move is to get the point across without going overboard. Round things to the nearest bill, and then starting thinking about how good the service was. $20 minimum, period.
Tipping is something that "Your People" can't really do -- it doesn't come off as personal -- so therefore do it right.