For 3-4 years, I grew 6 trees with stepover pruning. I saw no bark damage until last winter, when voles stripped all the bark from the trunks and the vast majority from horizontal branches. [I know it was voles (or similar) because only a small rodent could burrow under the protection and set up nests there. Also, there were a million tiny teeth marks -- death by a thousand cuts.] It seemed very evident that they ate the bark while climbing the trees; damage extended up to 2' above ground and was more extensive on the upper surface of each horizontal branch.
I don't think the stepover form presents special risks. A vole that completely girdles the trunk on a tall, central-leader tree will kill it just as reliably.
Meanwhile, there had also been modest fruit loss from rodents. My guesstimate is that the worst pests for the fruit are (in descending order) raccoons, possums, squirrels, birds (maybe especially catbirds), deer, and then small rodents (mice, chipmunks), followed by rabbits and woodchucks. I have all of those here in abundance (but no rats, as far as I can tell). Small rodents and rabbits tend to focus on lower fruit; the others will take it all.
FYI, I coppiced my 6 damaged in-ground trees and they are recovering; 5 of 6 are regrowing strongly. Whatever pruning method I use, I will think more seriously in the future about protection from voles.
p.s. Like Mike, I have also seen a "groundhog" climb a peach tree. It was a drought year and I think the animal was desperate for water.