EDIT: Summary of my comment: noticing whether you would get caught costs a lot of mental energy that would likely be more productively used elsewhere.

On one hand:

• You get to do something better with the money. Given that possibility, it's wasteful to spend it on something like tips instead.

• Most people (at least in the US) tip, and therefore are motivated to come up with arguments for tipping. This means we have probably heard more arguments for than against, relative to the true merits, and should discount the case for tipping appropriately.

On the other:

• Getting caught sends a terrible signal to possible allies that you will defect when you think a rule is unenforceable. It's hard to know when you will get caught, so the mental energy required to track likely consequences is likely to far exceed the value of the money saved from not tipping.

• 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.

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

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.