I expect that this question has already been raised and discussed elsewhere, but my cursory search of the forums (before I hit the wall of laziness) didn't turn up what I was looking for.

I seem to recall reading somewhere that tartan flashes are not generally standard for formal (black tie) dress.
I am curious how strict a rule this is. Would wearing matched (to my kilt) tartan flashes in a formal wedding party be considered a faux-pas?
(bear in mind as well, that the wedding party is likely to be renting their outfits, and the members have the leeway to choose their own tartan of preference).