Hey guys, kind of an oddball rules question here.

I have a box of proxy decks for Vintage. I'm not super familiar with all the lists I have proxied up, so sometimes after boarding, I can't remember which cards were in the main and which were in the side. This becomes worse when I lend the decks out. Would it be against the rules (or, for that matter, unsportsmanlike) for me to sharpie "sideboard" on the sideboard cards?