Again, reason I'm wondering about it is to know whether I'll have to worry about ending up in already-visited worlds...
I'll find out soon™. I think I only have one or two more exclusive UM legends to go. Then, I'm going back for a (hopefully) 6IV Mewtwo; I have a 5IV, but it's missing SpA and I don't want to use Mewtwo X. So I'll test a few thing in-between.
At least they made sure the lowest you can get is 3IV.