There are many areas in the wild where interbreeding occurs, but being a purist, I don't like to see it happen in captivity. I have several different sub species of box turtle and each species is in its own habitat.
They typically prefer to mate with their own sub species, but if none are available they will interbreed. I personally wouldn't do it as the animals I want to produce are the ideal representative form of each sub species. I've recently seen some Ornate x Eastern and some Florida x Eastern hybrids and they exhibited traits of both sub species, but they were "watered down" and weren't as beautiful as turtles bred with their own sub species.