Human beings often value deontological or virtue ethics, not consequentialism. You will feel (and therefore act, i.e. give of signals) like a bad, guilty person. This effect will vary a lot depending on your disposition. It also might be a poor trade for your utility function, again varying by individual values and dispositions.

It might also motivate you to be extra careful to use the money well so you can justify your actions.

TDT-like considerations, a desire to gain cooperation from other TDT agents on single-shot prisoners' dilemmas.

Most waiters aren't TDT agents. TDT's prisoner's dilemma algorithm is to cooperate if and only if your opponent will cooperate if and only if you cooperate.

I consider only my first argument for tipping to be a strong one, but on its own I think it is strong enough that I would be quite surprised if a rational sociopath (e.g. Quirrelmort) failed to tip.

Agreed.