Another way that I protect my edges in updo styles is to pin them back instead of pulling them into the elastic, like this:
How do you protect your edges?
I don't wear many buns anymore, but when I do, I am mindful not to pull my hair too tight (I pull my hair through twice, not thrice, tee hee) and I often use my hands to smooth (instead of a brush). Tying my wet and buttered up edges down with a scarf overnight, smooths them out with little effort. I'm also careful, when wearing a plastic baggy or bonnet with elastic, not to let it ride up onto my hair line.