I've been dipping longer than I've been officiating, but I can't imagine chewing tobacco and having a whistle in my mouth at the same time. Seems like the shrapnel would clog my whistle up, wouldn't it? I've never seen a ref chew while working a match, but I'm sure it happens. It's not that hard for a coach (or official) to just go outside and take a few minutes for a dip.