I recommend getting one at 12 weeks. Not any sooner than that-unless there's bleeding of course. 12 weeks seems perfect to me because you can actually see the little toot and hear a good heart beat. Also, it's nice to know how many babies are in there lol.
I didn't get an ultrasound done until around 13 weeks. I think as long as you aren't bleeding and you feel fine than you should be fine. :)
If I didn't have any bleeding I wouldn't have had one until I was 20 weeks...