Regarding Dan Malleck's suggestion that tipping & round-buying may be
related -- this sounds good to me.  There's a lot of evidence that
Americans bought rounds in egalitarian health-drinking ceremonies.
Basically any man joining a drinking party had to remain until everyone
had bought a round for him, at which point he would be expected to buy a
round for everyone else (and would then be free to leave).
 
Americans sensitive to the potential indignity or inequality of service
might want to include the barservant.  Maybe tips emerged as an
alternative to buying the servant a drink?  It could be a really long
night for servants unable to honorably leave the bar . . . .
 
Jon Miller
U Iowa