It's true that an individual bar waiter's share of your Cheers gratuity is quite small, but they also get a share of everyone else's gratuity. So it comes out in the wash. That's the beauty of a pooled gratuity system.
Tipping extra above and beyond the automatic gratuity is entirely up to your individual wish. While in the past yeah I think I got better service because I tipped extra, I'm not sure that's as true as it used to be.