I have a probe set, but I've just never used them for ball pythons...once I learned how to pop them properly, it just didn't seem necessary.

I have had at least one hatchling with smaller 'bits' that it took me 3 or 4 checks to confirm the gender on, but I check them all numerous times before they go out, to be sure.
I haven't had any issues popping older snakes, either--in some cases it's actually easier.