Right now, you can wait until the end of the day and you can still buy the full day-- you don't have to commit first thing in the morning. On the app there is a place to link photos to your account-- the photographer scans the link on your phone and the photos go into your account and are watermarked unless you purchase.
I was going to buy the full week because we were going to be there 5 days, and I am really glad I did not. There were not a ton of photopass photographers out around the park and outside of OBB, most of the characters do not have photopass photographers. The lines were so long for the photopass photographers that were there, that we took less than usual and I wound up not buying every day. I'm usually really big on photopass. I was very irritated that we never got a picture with the big Mickey pumpkin on Main Street- but every time we were in the area it looked like the line would take an hour, or maybe even more, and I wasn't willing to give up that much time.
Our OBB party tickets came with photopass that day, and if you are on a park on a regular ticket before the party that day it also will give you all those photos once your ticket has been scanned for the party.
It was very nice to be able to see what photos you got for the day and then make a decision on whether you really want to buy that day. I would have wasted money if I had bought in advance.