I had a gender scan at 14w5d and it was accurate. It was so obvious at this gestation what the sex was, and each ultrasound since has been the same. The lady I went to said potty shots at this gestation can be very accurate, you just need a good tech who knows what they are looking at/for, and a decent machine. The place I went to does gender from 14 weeks, and gives a 99% accuracy.
I think a lot of the time, the patient is desperate to have a look and see if they can tell the gender. So the tech does so at the patients request. I know when i went for my nuchal, i asked her to have a peek between the legs, knowing full ll that at 12 w it was completely 50/50. But she did so anyway, becauseni had asked her to. As expected, her guess at this stage was wrong, but it was me who wanted to look, not her.
Whether or not gender is determinable at this 14/15 weeks depends on a lot of different things...I guess if the patient wants the gender guess to be more accurate, they should wait until their 20'week scan, and not even worry about the gender before then..!