I had this same question a few weeks ago. Basically, when I called my OB office to check on the amount. They simply told me to drink as much as I could and they realized/said that I would most likely not be able to drink the whole 32 oz and try to hold it :) They said a full bladder makes it easier for them to see the baby.
On my ultrasound day, I guess b/c I was so nervous but I ended up going the bathroom before I left for the appointment and once I got there :) b/c I couldn't hold it. Bad I know! I brought a water bottle with me and drank this on the drive there and while I waited to go in. Despite going the bathroom immediately before the US, they never said anything about not being full enough and were able to get all of the measurements needed. I was even asked to go the bathroom at one point during the US b/c the lady was not able to see if it was a boy or girl.
Bottom line- drink as much as you comfortable can before and on the way to the appointment and dont' necessary worry about getting in the exact 32 oz