Another thing about proper tourque...When the bolts are tightened properly, the friction between the bumper & frame horns will take a lot of the forces. To loose and the shear resistance of the bolts have to take all the load. To tight and you can fail the bolt by having stressed the bolt paste its yeild strength. By doing this the bolt will strain and "loosen". When you retourque the bolt, you strain it even further. Eventually it will snap.

So if you continually find yourself retourquing these, it may be time to replace the hardware.