I am probably the meanest so-and-so when it comes to buying rounds, but I keep doing it, as bad as the worst described in this thread. It causes a lot of rows with my OH, somethimes I wish she'd chuck me over it (and my many other faults). It's not like I can't afford it either...
I remember though when one of my friends was unemployed for a while: he absolutely refused to be bought drinks as he could not afford a round in return, so he bought his drinks independently. It seemed odd but it worked...