I come from an East London family and the rule is simple: always try to pay a bit more than everyone else, and you will be OK! :)
If I am out with my husband and a single friend, we ALWAYS, without exception, buy a round each. To do otherwise would be really tight and in the culture where I'm from, would mean that you were ungenerous in spirit.
Also, if I am out with a friend who I know has less of an income than we do, I make sure it is always us that buys the final 'odd' round. If one of my friends is unemployed or in difficulties, I try all kinds of subtle ways to make sure that they don't pay very much.
I love whisky, but it is very expensive to drink good stuff. Plus, I like doubles :) I ask for wine when it is someone else's round, and only buy whisky when it is my round or my husband's.
However, some degree of reciprocity is important and generosity of spirit is important to me in my friends. I rarely drop friends, but I did stop seeing one who freeloaded off me all the time (he NEVER bought a drink, and was constantly 'borrowing' money that was never returned - he 'needed' £300 for the electricity bill, but could afford to spend £600 on rare vinyl records).