The amount of wear put on bearings and balljoints is the same regardless of whether you have a wheel that has 4.5" backspacing, or use a wheel with 6" backspacing and add a 1.5" spacer.
The increased wear is simply due to leverage. It doesn't matter what method is used to reduce that backspacing - it's just a matter of how far "out" from the bearing the force is exerted.
I don't run spacers myself, but from the reading I've done, there are two things that I've seen to watch for.
#1. Get a quality spacer that comes with lugs on it. The ones that try to fit between your wheel and the hub are not something you want on your rig. The pic that swampsinger posted is an example f the type that I'd look to run.
#2. Torque wrenches and loctite are not optional. They're required. Then agian, torque wrenches should be used ANY time you put a wheel on, spacers or no.