It might become audible, maybe even bothersome to some but I would still consider it to be largely a non-issue. In the case of quad simply by moving the rear speakers more to the sides there is no real cogging effect that I notice.It becomes an issue when the mixer tries to smoothly pan over a side of the room, where the sounds from different speakers mainly reach to one single human ear (if we don't turn our heads).
The same idea holds for 7.1, simply move the side and rear speakers a bit farther forward to reduce the "cogging". Other than gimmicky effects with music and perhaps sound effects in movies I don't think that it is really worth worrying about!