I replaced all 4 of my dust shields when I had to change all 4 wheel bearings, perfect time and access to do so. The ASE master mechanic at my work said they provide a certain distance tolerance from the wheel bearing hub assembly to the axles on the front. As for the back, the distance from the rotor to the axle. If the dust shields aren't on, he said the life of all the wheel bearings all the way around would prematurely shorten.
As far as adjusting the existing, you'd have to remove them and flatten them on a flat surface & just keep test fitting them. A lot of work to do if you're not replacing them. Either keep bending them with a pry bar or screw driver or cut them off until it's time to replace wheel bearings.