I have a fairly simple set of rule regarding tipping (or more importantly when not to tip).

Changing the price from that agreed, if agreed in advance = no tip, and original price paid.

Refusal to use meter where fitted = preferably don't use them at all, but if not feasible no tip on top of agreed sum.

Dishonesty/incompetence, e.g. taking roundabout routes = no tip.

Rudeness, either to me or other drivers = no tip.

Asking for a tip, or gesturing for one, or in any way referring to one = *definitely* no tip. I will *not* be pressured; it is my discretion and my decision.

I wish more people would do this, then they might learn.

Neil