I've had my 770 for over 2 years now with no need for a screen protector. I have one 1cm scuff where the protective cover bent and touched the screen when sliding it off (oh the irony!) but from normal use the screen is fine. It all depends on how heavy you want to use the tablet. If it's daily and doing a lot of onscreen keyboard work, for sure.

Long story short(er), it couldn't hurt. Worse it could do is protect your screen. There are a lot of threads on here as to which is the best, check those for recommendations.