I always want to see the airplane upside down at least once with the student in control. I'm not too picky on how they get there, but I want to see an exit back to upright flight in a controlled manner. If the student wants to do a spilt-s, that's fine. I tend to teach a loop and roll, then a split-s just along the way to "spice things up", so it's usually a non-issue. But if a student can't handle being upside down with out panic, they aren't ready to fly un-assisted, IMHO.