In 20 years I've never raised the subject of payment while in the bar. Very occasionally I'll be asked and it's generally not a good sign. I only ever had one problem, over 10 years ago, when an off in Bangkok was unhappy with 1500 Baht for short time. He wanted 1700. I refused and he sulked.
Having said that, my expectations of short time tips have had to be adjusted, having recently returned to Bangkok after a long absence. It's clear that the boys in the major bars expect 2000 Baht for short time.