Here are my rules:

1 it is the responsibility of the employer to may an appropriate living wage.

2 there are very few cases where a tip more that 15% is warranted

3 never tip in advance before you have received service

4 never tip when it is not clear what you are being appreciative of

> self-checkout machines

I'm sorry, what? What sort of self checkout machines are bow requesting tips?

> The idea is that workers earn a subminimum wage and then customer tips make up the difference to get workers up to minimum wage.

This is really the root cause of all this toxic American tipping behaviour. They should get at least minimum wage, and the tips on top.