None at all. The thing stop does is stop the developing process quickly, which makes consistancy easier. There can be some development that continues once it's in the rinse, but as long as it's consistent you can account for it. As I understand it, stop also helps preserve the fix by not lowering it's ph with any developer carryover that may occur (but with a good rinse, it shouldn't be much of an issue). But then again there are also alkaline fixers, I'm sure someone who who understands the chemistry in more detail will help out with facts about that part.