The only "rule" that I won't break:

If I'm not getting married, I won't don the fly plaid. The fly plaid is for the groom, and the groom alone.

Other than that and this will come up this summer, I try to dress at least one notch more casual than the groom. I have a friend that is doing a kilted wedding this summer, and I wouldn't show up to his wedding wearing a PC if he is wearing an argyle, I wouldn't wear tartan hose if he is wearing aran hose, etc.