I doubt they’ve had the need to ever do this. Dean doesn’t worry about himself too much, going with the “If it ain’t broke, don’t fix it” mindset, and Sam isn’t particularly interested in looking into a mouth he could fit in. If he absolutely needed to help Dean out, he’d do it, but that’s about it.
Of course, Dean might tease Sam by baring his teeth and going “Something in my teeth?” if he catches Sam giving him any odd looks about this.
Brothers Apart 