I believe that you're better off with the bumpers stuck to the paint......they will easily peel off and any adhesive residue will come off easily with WD-40 and a paper towel. If you put them on the back of the deflector, the paint could get worn off from the vibration of the deflector at high speeds.......at least, that's my take on it and that's how I"ve always done it on my vehicles with good results.
Jim
PS........I now see that KCFlyer beat me to the punch........aha! I was right!