Yes, that is a tricky one, I find.
First of, guys do buy more rounds if you just leave things the way they naturally go. I don’t think they like spending the money, but reckon they probably get something out of the “bread winner” feeling, if you know what I mean by that. This apart from/on top of the picking up thing.
As for your question:
Most of the time I stay out of the rounds altogether to avoid the problem, or I might have a mini round thing going with a female friend of two.
I will occasionally accept a drink from the guys when it’s offered as friendship-cement, which I find is often the case.
I will also accept a drink from people (male or female) who I know are significantly better off than I am.
I will also accept them, unconditionally, if I’m playing music in the pub from anyone not playing. I play for fun, but getting a pint or two bought for you goes with it.
As for buying them. I say, once in a while: insist. I think it is acceptable to buy fewer rounds than the guys, but insisting to pay once in a while will probably be very much appreciated. In my experience guys will buy rounds, even if they can’t really spare the money.
Mind you, I have known the odd guy to actually get offended when I insisted. But I think it’s better to err on that side than to be thought of as mean.