We started ours after we found out the sex. All of our big ticket items are neutral, but the clothes/blankets etc.... are boy colors. It was easier for us to wait so we did not have to go back and change items to more gender specific colors.
I registered at about 19 weeks, I didn't wait to find out the sex. We are keeping the sex a secret too, so finding out didn't matter. We decided to do that because it seems like every baby shower I attend where the girl told what they were having ended up getting so many baby clothes in that gender color it was sad. I'd rather have necesseties, like bottles, wipes, etc... than a bunch of newborn sized clothes the baby will grow out of in a week.
If you can, I would suggest starting your registry online. There are plenty of things you can register for now before you know the sex (we haven't found out the sex, so that wasn't a factor for us). However, the first time I went into Babies R Us with the intent of picking out stuff for our baby, I left feeling VERY overwhelmed and quite depressed. I was very grateful that I had done a lot of it onine before going to the store.
Re: Registering question?
We started ours early because family wanted ideas for Christmas.
Oh--and for us, waiting until the big U/S doesn't matter because DH and I are keeping it a secret from everyone else!