I do not. My old pony I bought this spring was rode by kids and learned that she can just pause and take a bite whenever. It's annoying, and it's taken me a long time to break her of this habit (still working on it, but much better). Whenever you're stopped, she'll take advantage of the situation and if the grass is long enough, reach over and grab some.
My friend and I joke that she has two emotions: Hungry and Apathetic.
My younger mare, on the other hand, was never taught that it was possible to eat while being rode, and it's great having a horse you can ground tie on grass and that will stand there without looking around to try and find something to eat. It's great that she doesn't yank your arm off, either.
Plus, it's nice to have a clean bit not coated in a slimy green goo..