I've heard that 27-32 weeks is the best time but that can slightly differ depending on where you get the information. I think it's because the baby has more fat after 27 weeks, but after 32 weeks you're less likely to get a good picture because there's not as much space in there. I could be wrong though, this is what I've been told/read.
We've had two 3d/4d scans so far, at about 19 weeks and 24 weeks. That's not usually recommended but I'm so glad we did!