One of the things that frustrates my students and consequently me as a teacher is how often a successful thumbnail drawing is abandoned in the sketching process. I will often look at a good thumbnail and then, when I see revised drawings, wonder what happened to the flow, design, rhythm, etc. that I remember from the first sketch Granted, some changes may be intentional and for the better, but if you lose the essence of the thumbnail- that thing that attracted you to the design and got you excited in the first place- then you are essentially starting over.
I always thumbnail out my idea first, before taking any photos. This allows me to solidify in my mind things like angles, pose, lighting and basic composition. I then use my sketch to dictate how the photo shoot goes.
I match my models as closely as I can to the thumbnail idea. Sometimes the photo shoot suggests other options, and it's a good idea to take those photos as well, but I rarely go into a photo shoot without solid sketches.
If that happens, then I am letting the camera dictate the design of my piece and not my imagination. I think my imagination is usually superior to the mindlessness of a camera, so I try to not let the camera decide things for me.
I also take lots of detail shots as sometimes the angles and poses in my thumbnail are not comfortable or realistic for my models to capture in real life. I use the detail shots to get the info I need. I then take my thumbnail, lay tracing paper over the top and make my final drawing using details from multiple photos. Failure to use tracing paper in this stage of the process is the downfall of many a good student thumbnail.
I enlarge my thumbnail and use it as the "bones" of my design. I add the details or the "muscles and skin" over the top on the tracing paper. I can still see the original design and thus I can follow it, preserving the space relationships, rhythm and compositions that I carefully created in the thumbnail sketch.
If I had abandoned my thumbnail during the creation of this piece, I would likely not have been as happy with the result as I am. As you can see from the photos, none of them match my drawing exactly. I had to rely on information from all the photos as well as my thumbnail to arrive at the final sketch. There were even modifications in the final painting stage that were made to reflect more accurately the vision I had in the thumbnail stage. Trust your thumbnail. It usually does not lie because, if done well, it contains the bare essence of what you hope to express.
There is a great book by Ron Schick about how Norman Rockwell used reference photos called Norman Rockwell: Behind the Camera. It shows how Rockwell's process very closely mirrors what I and countless other artists use to get the information for their paintings.