Here where I live they recommend that you have your 3D-4D ultrasound done between 28-34 weeks, but they prefer to do it by or before the 28th week the reason for that is so that the baby still has room to move around and isn't too smashed. Also I would schedule your appointment as soon as you know that you are going to have them done, since they get booked very fast. I actually already scheduled mine when I was only 19 weeks pregnant. They also told me to drink plenty of water a week before my appojntment to insure good pictures.
most 3D places will schedule for the weeks they feel will get you the best results...i think after 28 weeks you should be able to get some really good images of the face...i also know you dont want to wait too long because the baby only gets bigger :)
I answered you on your post in 18-34 :)
http://www.prenatalpeek.com/calendar.html