Yep, they'll never give you dirty looks, they'll just talk about it when you're gone. In the Phils it's a different story, they'll call you cheap as you leave.
If you go back to bars on a regular basis then I agree completely, it pays to keep those staff happy. I dont normally tip much, just the change (20, 40 or 100 B if the bill is high enough) and I dont seem to have problems with the bars I go to regularly.
However, tipping is only really expected in tourist towns like HKT, CM, Patts and BKK (kinda touristy).