My letter said to drink 8-12oz 1 hour before the appointment and not to void.
I was about to burst by the time i got in to my scan (40 minutes late as they were running behind! torture!).
The sonograoher told me she only needed a full bladder to see my ovaries and uterus properly and as soon as she had got the right images, she let me go pee. She said she didn't need a full bladder to get images of the baby. She then spent the rest of the appointment complaining that because I hadn't eaten lunch, the baby wouldn't move and that she couldn't get the right images. she eventually gave up and I had to come back a week later so they could get the proper images of the heart, though I don't think that had anything to do with bladder content!