The screens are pretty hardy due to the materials used. The screen on the aluminum version is Ion-X Glass (similar to Gorilla glass) whereas the screen on the stainless steel version is sapphire crystal. While both offer scratch resistance, sapphire crystal offers better resistance than Ion-X Glass.
Despite these screens’ inherent scratch resistance properties though, if you bump and scrape frequently, you may want to consider a screen protector. I have used one on my Series 2 aluminum almost from the time of purchase over a year ago. I have no scratches or scrapes on my screen. The protector is barely noticeable, and does not interfere with usage. Except for the guy at Apple who put a new band on my watch a few months ago, no one has ever noticed the screen protector I am using. (Note: nothing went wrong with the original band, I just wanted another color option.)