I consider the class I am travelling. Flying coach, I usually don't ring the bell because I think they are pretty much understaffed. In addition, I always try to get the lowest possible prices, so I accept that the FA's are not my personal servants.

Flying business, I don't have any scruples. The ticket costs a lot of money, and there are enough FA's around.

The reaction of the FA's depends on the airline. In asian airlines, the FA's seem to be embarassed when you walk over to them to get something. It's just the opposite with european FA's.