Be aware that even at the set locations there will be times when they aren't there. Say for example at the Partners statue in the hub. When the PP photog has to take a break there may not be someone there. During the busiest times sometimes they will have someone to fill in, but not always.
Just like when you are at WDW you will want to be on the lookout for the photograhpers. Make it a point to stop at everyone and, if you want magical shots, ask every photog if they take any. They won't take any at night, once they have set up their tripods, BTW.
Also, if you are planning on buying the CD let them know. They usually only take 3 photos, but if they aren't busy and are good at their job many times they will offer to take more for you. Sometimes even moving you to a slightly different location.
Recently we were at Paradise Pier in DCA by the Toy Story Meet 'n Greet. There was a CM and a PP photog just standing there waiting for the characters to return from break. I asked her is she could take some pix and she took quite a few of us, happily. We did something similiar this past holiday season when the photog was waiting for Santa to come back. The characters have more scheduled breaks than the photogs do so many times they will be waiting around for them to come back. On Paradise Pier where they take pix with Duffy there are many times the photog is just standing there, alone.
During the day there may be a roving photog taking pix for the Memories show on IASW. We haven't run into one yet but we've heard they exist.
Have fun!