We just had this issue in action today at lunch with my In-Laws (she is a very cheap tipper, she's usually in the $1 per person club). We went out with them, and she asked for separate checks for FIL and her (which irritates me, but whatever). She went to the restroom and the bills came, so FIL paid it. Their bill was $26.83, and my FIL tipped $6.00 which I thought was about what I would have tipped with their bill (close to 20%, and do some rounding to not complicate things). As soon as MIL came out from the bathroom, she started on FIL that he tipped "WAY too much". She told him that he should have tipped no more than $3.00.

Our bill was $40.83, I gave the waitress $49.00 (I would have given her an even $50, but I didn't have the right bills). And I was tempted to leave extra beyond that because both of the in-laws are a major problem when we go out to eat (but my thoughts for that belong on the "Things you wish you could say out loud to other people" thread.