I don't want to fiddle around through a machine menu, so if it has reasonable suggestions like 12% 15% and 18% I'll usually choose one of those.
If their automatic suggestions are at something like 20% 25% 30% I usually just press no tip instead of custom and entering in another value.
I justify it by saying the next three people paying 25% are paying for my 15% tip, because obviously they can afford it more than I can.