My old Edge 1000 always had a screen protector + rubber case, and so will my new 830 as, even though I now do 80% of my riding on the road, they still get chucked into boxes in the back of the car, or dropped on the ground in car parks.
I'm using a tempered glass protector from Amazon and it fits and works a treat, with no degradation to the touch screen sensitivity. I swapped to glass from film ones on the Edge 1000 too and they last longer before going tatty. Highly recommended.
I bought a tempered glass protector from Amazon (like $8 for a 3 pack) and it works great on the 830. At dirst I thought it was a little bit undersized, but then I purchased and installed the silicon case and it fits perfectly with the silicon case in place.
I decided to go with the tempered glass protector after taking a look at the screen on my old Edge 810 and seeing permanent scuffing marks on the screen.
I have had 4 Edges going back 10 years using them on the road and mtbing in dirt and mud. Never a used screen protector or silicone case on any of them and they held up well...maybe an inconsequential scratch or two if you looked really hard, but that was it. The 830 screen doesn't appear to be any more susceptible than those, so I would say no, you don't need one. It can't help the visibility of the display to add additional layers over it.
I've definitely seen other people with older Edge devices that have been used for MTB or CX (especially the latter in the winter!) where the screen has turned distinctly opaque from the myriad of small scratches all over the display. It seems that a combination of dirt + using the touch screen was the culprit, so I suppose it will depend on how you use the device, but I know that I would still fiddle with the screen even when it's coated in abrasive filth, so a screen protector is definitely a good thing for me at least! I suspect that the 5xx series (not having a touch screen) would be far less susceptible to this.
I'm buying one and a silicone cover. Had one on my 800, 1000. The glass ones are inexpensive and once applied you can not tell they are there. As for the cover, these things are easy to drop. One slip and you'll be happy. Get a bright ugly color you can find in the leaves on the side of the road or trail.
