I do dismount him when I allow him to graze. If I have him at a stop or if I'm giving him a breather after going up a long steep hill, I keep him away from possible eating areas and keep his head up so he doesn't get tempted.
THIS is probably the best thing to do--make it clear when he can graze and when he cannot.
Horses' guts are designed for 8 small meals/day. If you go on a very long, like all day ride, you'll need to either bring forage, or stop for grazing during the day, or else the bacteria and stomach juices will have nothing to dissolve. SOME
horses develop ulcers bc of this. I don't believe that they should be stopping whenever they want to eat, just like I don't believe that my horse under saddle should stop to defecate since I see them often (in my back yard) defecate while walking, trotting or cantering with the herd. Movement helps a horse to digest, just like it helps keep us...um...regular--Especially relaxed walking, like on a trail ride.
I'm changing how I trail ride next year. DH and I are planning on a week long, SD trip. I'm bringing my horse's feed bags--they look like this~
Cavalry Feed bag (w/leather on the bottom, leather venting and harness leather crown piece) Civil War Cavalry Saddles, Gear, & Accessories
and filling them with measured out alfalfa cubes, so that we can stop a few times and give them "2nd breakfast" and a lunch break next to a water source.