I hardly ever carry small cash (or any cash) so I hate the scramble for singles or fives at the end of a tip-required interaction, as with bellhops or valet parking. I don't mind tipping for room service, dining, bar, etc. if I can add the sum to the tab, but I don't like having to keep money in my wallet. I do avoid bellhops and valet for this reason and would rather carry my own bags and park my own car anyway.