In addition to blind curves, there are hill crests. It happens fairly regularly to me. It depends on the situation. If they're going to pass, I'll slow to allow them to get by more quickly. Usually I'm ahead of them and can see further around the curve or over the crest. If I see conflicting oncoming traffic, I'll brake hard to allow the passing vehicle to get by before they pull over into me. And sometimes I just cringe and hope nothing bad happens.