My personal opinion is as long as you have something to protect it in situations where it may be in close proximity with objects that would rub against it (such as having it in a suitcase while traveling) there is no need for a screen protector. With a phone I can understand having a screen protector because of it constantly is going in and out of pockets and who knows what could be in pockets that could scratch a screen such as coins, keys, bits of sand, etc... That being said, my Sprint Epic 4G has been without a screen protector for over a year, has been dropped more times then I care to admit, and without carefully examining a freshly cleaned screen under a bright light, there are no scuffs or scratched to be found.
A screen protector isn't going to do anything to prevent a shattered screen from dropping it, so if that is a concern I would go with a case to provide some padding.