I wouldn't use a fence ring for any apple incapable of growing above the browse line. A dwarf orchard should have its perimeter entirely fenced, unless it is an unusual situation where the trees are scattered about the property. Then you could fence either individual trees or clusters leaving enough room to accomadate the trees when full grown. For most cultivars, M26 is as small as you can possibly go to get a tree above the browse line.
[NAFEX] fence rings for dwarf apples,
Alan Haigh, 06/07/2009