There are two minds of this. Heather at Rabbit Haven says that NO nipping should be allowed. The moment one goes for the other, it is seperate and put the nipee on top of the nipper within 3 seconds (apparently if you don’t dicipline them within 3 seconds, bunnies don’t feel they should be held accountable)
Meanwhile at the House Rabbit Society, they say that nipping is part of the dominance issue and some nips are fine. And there have been several people who did the bonding process by saying, “You know, I’ve been at this for weeks now and you two still haven’t sorted things out. So I’m putting you both in an x-pen until you do.” and let them go at it. I’m not a big fan of that last one however, I did allow a fight to go a little longer than usual as Dono began to fight back and that seemed to have some effect on Maryann not pushing him around for a while.
My personal thought about it is if it is a single nip (Like Maryann is prone to do), it is no big consequence. And sometimes the mounting issue is followed up with the top bun nipping the back of the neck – they do this often in mating as well. So I’d say that those nips at the neck aren’t too dangerous as evolution has given them the fur there. However, if you get them doing the spinning Yin/Yang, that is complete out and out brawling and those nips shouldn’t be allowed. Also, nipping to the face, eyes, nose or ears (basically anything above the neck) should be immediately discouraged as there is sensitive areas there.
Usually a quick nip isn’t much to worry about as buns have fur just for that reason (It’s amazing how quickly Dono ‘sheds’ the mouthfuls that Maryann gets). However, if they seem to be dining on one spot, or nip the face, it could cause injury.
As always, take all of my bonding advice with a grain of salt. Afterall, I still have two buns unbonded after 3+ months. 