As long as you're not bouncing and jouncing her when you ride, then bareback is safe for her.
I don't know if this makes a difference, but when we ride bareback we sit the trot. If you can post bareback, I admire you, :)
And collection is very important. If you let her throw her head up while trotting it will definitely cause more problems for her back. Otherwise it should be fine.
Bareback can accentuate balance problems, so you find her going funny you might wanna check your seat. They can feel every little thing bareback!
Obviously- if she shows signs of pain or has swelling, then stop, and maybe talk to your vet. But done right- you shouldnt have any problems!