How about you want to make a dark moody image (say just to test you ability to get detail in blacks) so you've got a preconcieved idea what the print is going to look like, but you haven't actually got the subject yet. You then go out looking for a subject that might fit your idea of print. A photogram might also fit into this category where you know what you are trying to do then go insearch of the subject to do it to (eg. a plant that's not transparent enough so you go and grab another one from the garden that looks more promising)
Pesonally, I'm a subject 1st person. I usually just wander around until I see something that looks interesting (to me) and snap away. Printing is generally a pretty straight reprensentation of the scene (as I remember it or as I decide to interpret it from the proof sheet). Even though I haven't taken many pics recently, I still don't get enough darkroom time to catch up with the backlog of 'maybe' pics, so I don't deviate much.
Good and Interesting question!