Well, now you know better than to ask.
In your shoes, I think I'd try to do both as informally as possible. It's not anything you can hold others to, but at least you'd have something written down to refer back to. That's kinda important when you have to go home and sleep and come back the next day on occasion.
If you are caught doing it, just tell the truth: You have to do it. Design in particular is an inherent part of any creative activity. My father-in-law (an ex-English teacher) creates outlines and rough-drafts for his vacation postcard messages. No matter how small and boxy the building you are sitting in right now, I guarantee you that somewhere there exists a blueprint for it.
Some activities are so trivial that you can come up with a design and keep it in your head without much effort, but most programs that do something useful will not be in that trivial category.