rzr lover, I mainly ride trails and woods, so the majority of my time is spent below 25mph. Since my S 900 can run almost 40mph in low, I spend almost all my time in low.
The main concern regarding gear selection is the load or work your machine is doing. If you are really working it with some technical hills, mud, snow in AWD, etc. then use low. I could easily do about 50 percent of my riding in High gear, but there are many advantages to using low, it will put less stress on your belt, it will create less heat and move more air; as someone already mentioned. I believe Low allows your primary to move through a greater range, which is a good thing.
If I know I'm going to be cruising open areas, then I use High, even if I'm at 25mph. There isn't much load in that situation and High does reduce the engine RPM's.
The only wrong gear is High for all situations, you'll be reducing belt life or worse. Now, you could run Low for everything, even cruising the roads. There is nothing wrong or damaging* about that, but most guys do get bothered by the high motor RPM's when cruising in low and the poor gas mileage. *Extended high engine RPM's could reduce engine life.
You should experiment, that is what I did. My last machine would not allow any cruising in Low gear, it was too low for that. So I've had to do a little experimenting. I actually prefer Low gear for almost all my riding now. At first I did the High, Low, High, Low shuffle and now I am dancing to a new tune LOL. Have fun and I hope this helps. :ride: