I've been thinking about this, and my rule of thumb seems to be that I'll tip if I pay for the product/service after consuming it. If I walk up to the counter, buy the product, pay, THEN get it, I won't tip.

My logic is that tips are supposed to be extra, based on quality - how can I know if I want to tip before I taste the coffee?

Then again, I'm from the UK - the culture is different over here.