The fryes I currently own (Veronica shorties) have rubber over the leather already. However, I have had other frye styles in the past with only the leather sole and I would definitely recommend having the rubber added if you do any sort of significant regular outdoor walking. They're thin and shouldn't affect the balance much. Depending on the design, it might also be useful to replace the heel plates with heavier ones with better traction - these tend to be a little thicker and would balance out the additional rubber in back. My commute (about 1 mile each way over sidewalks and cobblestones) destroys leather soles, plus I find them incredibly slippery when it's wet or icy.
