Some manufacturer's screens (or the cover) scratch easier than others. I've never had any one of them that I've kept for a good period of time (say 6+ months) not get scratched, even if it's just 'cleaning' scratches. It's cheap and easy protection on an otherwise expensive device.
One of the easiest ways to put on a screen protector...and not get any dust, scratches, bubbles, etc. (at least, it's worked for me the past 5 years).....
Clean surface thoroughly. Work in a high humidity area (bathroom with the doors closed and the shower running hot for a few minutes). Follow the screen protector installation instructions....but add one step. Spray the adhesive side 'very lightly' with ammonia free Windex before installing, then shake off the excess (like there's not enough for dripping). Hold the protector in one hand, peeled, with the Windex bottle in the other hand about 14-18 inches away. Install...and wipe the protector from center to outside edge with a credit card (or similar) gently to get out excess fluid. You may have to hold the protector in place. Then, immediate pat dry with lint free cloth. And....here's the hardest part. LEAVE it alone for a couple of hours, at least. Do not touch it, or try to speed the drying process whatsoever.
I use screen protectors made from the bulk material bought from bestskinsever.com. Several phones, several iPods, no problems.
HTH.