For the parts that really make sense to make reusable and generic, go ahead and do that piecemeal as those bits get used. For the parts that don't, don't bother with the up-front investment for very little return. Just do what gets you to the next part of your current project, and save notes on any really innovative bits in case you do need to replicate them. Tear down the scaffolding when you're done and the project is self-contained, and keep around the truly globally applicable bits on your truck.
Excellent advice which I felt should be highlighted. I'm always (it seems) struggling with this part, not so much because I don't create the notes, but because I have trouble locating them later.