John,
I have never sold one YET that has switched sexes. I quadruple check everything I send out, but I guess I haven't been diligent enough with stuff that I get in.
As for the ultrasound, I like to do it in January, to see those small follicles(10mm to 18mm), the ones I can't feel yet, so I can do some switcheroos if I have some males that aren't getting the job done. I gots plenty from 10-15, but not many that are palpable (over 20). Trust me, I do plenty of palpating.