I rarely sauce - serve it on the side - but like my ribs dry most of the time.
If I make tips, I do sauce tips.
To be honest, if I sauce, never thought about saucing the bottom of my ribs - just put a nice glaze across the top and let it drip over the sides.
I add a thin coating of sauce to both sides during the last 15-20 minutes. Not enough to make them messy, just enough to give them a hint of sauce flavor. I then have several varieties of sauce served on the side for personal taste.
I sauce one (top) side. Usually do 3 racks of BBacks and sauce 2 of the 3 for the wife and kids. I leave mine dry and apply sauce later. My wife likes Sweet Baby Rays that has been applied and thickened a bit after sitting for about 20 mins on the ribs while still in the WSM.
Like Michael said, it is a personal preference...I normally sauce both sides on the grill..I really like what high heat does to the finish, but only for 5 mins or so...then I take them in and add a glaze to them right before cutting them. Great...now I'm hungry again!