I also had friends who worked in that business and I always tip 20% no matter what kind of service I get. So, when you come back again they wil recognize you and always get better service than others on Friday nights, drinks come faster.
It depends on which Asian you are talking about. Tipping is common in places with US influence like the Philippines. However, in other countries, no one tips. Not sure if it is considered an insult - more like they are just not used to the concept. Maybe it depends on culture. I did tip this Chinese restaurant guy once and he vehemently refused it. (This was in the US) I was like ok…
No tipping in KR thankfully, and the service is always great. On business trips to the US the service either 1) sucks or 2) borderline, yet these people expect a tip. The food is already double what I would pay in KR and 1/2 the quality, then they add a tip on top, and and tax too!