I think that it would be REALLY hard to see and determine the angle of the parts at 12 weeks.... At our 12 week U/S you couldn't see anything at all that had to do with genitals....
LOL, it's ok, I wanted to find out ASAP too, but HONESTLY, now, it seems like we found out SOOOO long ago that we were having a boy. You will have plenty of time to plan everything out after you find out for sure!!!
I've heard of it, but I feel like it would be hard to figure out if you weren't used to looking at u/s all the time. Heck, our tech was narrating for us what parts of the body we were looking at, and I STILL didn't see half the stuff she was seeing. "Oh, that's a hand? Okay. Still looks like Skeletor."