I saw this question on Quora and thought I would answer it here on PubClub, then post the link on Quora.
The answer is the same as it is in many other aspects of life: it depends. My expertise comes from being a nightlife veteran, from observing guys buy drinks for girls in bars, from doing it myself and from seeing girls and their reactions to having drinks bought for them.
In this case, it depends on your generosity, your income and your interest in the girl. If you’re doing it just to show off, then sure. If you’re doing it to try and pick her up, then you should be more picky and think about it first.
I will assume you want to pick her up and take her home that night. Just walking up and offering to buy her a drink will not get you anywhere. I recommend that you move in next to her and establish communication – start to talk to her and judge her interest in you.
Often she has a “watchdog” evaluating you. Often that girl is not as attractive as your target and will do anything she can to block your advances. (This is where a good wingman comes in handy to keep her occupied and not even this works most of the time. This has cost me getting laid I don’t know how many times.)
Back to the girl – I judge the situation and if she is receptive to hanging out and talking for a while then I have no problem buying her a drink. Just be aware that she could be a “drink and dash” girl. That’s why I take the slow approach rather than just charging in there.
Some guys do buy girls drinks almost at will and hope for the best. I don’t find this a very successful pickup strategy. Some girls actually expect guys to buy them drinks in a bar and those are not the girls that you want to hang out with because they will leave you the second you don’t buy them a drink. And often move onto somebody else.
Groups of girls, such those as a bachelorette party, can be fun and there’s no harm to buy the group a round if you can afford it. Just don’t expect any romance in return.
So yes, it depends. It depends on your personally, the situation and the girls. Just keep a watch out for those watchdog friends of hers.