I think the honest answer is that nobody here knows. At least, that’s what I’m getting from the replies so far (google it, buy a book, just experiment).
If I find out anything of use, I’ll post it here – maybe it can be useful for someone else with the question.
So far, all I’ve been able to find out that might be relevant to some people, is that youtube recommends a (closed, whatever that means, maybe the checkbox for fixed?) GOP of half the frame rate (so 12 for 24fps, 15 for 30fps, 30 for 60 fps). But I’ve also seen settings in presets ranging from 1 to 120. B frames seem to be directly related to the GOP rate, but again I haven’t found anything useful online so far.
I’m assuming here, but I think people would have 2 main specific times for wanting to know GOP/B frame settings. One is youtube/vimeo, the other is trying to get highest quality close to lossless, for archival or master copy. People who don’t seem to know the answer can tell you to look it up, use the presets, or just experiment. But video editing is already such a time consuming process – by the time you’re ready for export, you want to be done messing around.
Hopefully someone will share their experience or possibly even a useful layman’s explanation.