Using Velox, I pull it pretty tight. Not crazy, strain-my-@ass-off tight, but I keep it pretty snug all the way around. After a week or two, it dimples in where the spokes are, and shows the outline of the bushings, but still prevents tube damage.
Be sure you have the right width for you rim. It should completely cover the bed and may even look a little too wide before you put it in. Hope this helps.