If you have a primary use for certain images, it helps me to know before we shoot the scene. Instagram posts, for example, look best sized 5x4. 4x6 is used a lot on websites and for prints. 9x16 is a common movie format. When used as 16x9 it can accentuate height. While Facebook, Instagram, and Model Mayhem profile pictures need to be square.
If I know before we shoot a scene, I can set my camera to a specific aspect ratio. That helps me a lot in framing the shot and keeps cropping to a minimum.
4x6
4x6
16x9
5x4
5x4
1x1
Be sure to include and verify the model's pseudonym, Model Mayhem user number, Instagram username, and modeling web site URL so they can be added to the metadata caption field. That way, the model can be tagged on IG, credited on MM, and contacted by others.