Not had it happen near enough me to do anything about it, but if it were right above my head, and the ascent was relatively smooth, I'd certainly quickly unbuckle and close it. If we were bouncing around a bit and there were potential head-bruisers overhead, but I felt safe enough I have to say I'd do the same before I got a bruised head!

Yes, I agree - assess the sitch and act accordingly. There's no right answer for every situation.