Your buddy may have been confusing motor spray with brake cleaner, which does leave behind a residue (and it manages to smell even worse than motor spray).
I don't polish my comm's all thats going to do it through them out of round, if your going to be breaking brushes in on that comm right away anyways what is the difference. If the comm is polished it will just take a tiny bit longer for the brushes to break in. Not to mention if you accidentally push your car accross your pit table that polish is gone. So why waste your time...I suppose if you tested it on a dyno you would be able to find if there is any advantage at all, though I would guess it is unlikely.