I try to give the pass when the leader is in position to pass. I will give up position/line sooner if I'm being lapped. Never take a defensive line when being lapped. If being lapped and I see a car pulling out for a pass, I give a free pass. I don't want to go two wide with a car that's lapping me. There's no reason for it. The sooner he passes me, the sooner I can do my own thing, without thinking about when he's going to pass me.
The only time I challenge a lapping car is if it's a purely recreational/non-competitive race, he has a decent lead, and I have good pace. I'll only pass a leading car when I'm lapped if I have consistent or noticeable pace advantage, or he had an incident.
Like most, I try not to screw up anyone's race.