I think there is some truth to eating grass with a upset tummy. Jackson does it with both though. It's funny.
When we are outside, sometimes it seems like more of a nervous habit. If he's got a lot of energy and has been running and chasing my dads dog and swimming, etc, he immediately hops out of the pool and quickly starts munching on grass, like he's trying to release some pent up energy or something, it's weird. I always thought maybe it was the moisture in it, and his way of getting some? LOL I don't know. Then sometimes it's purely out of boredom, or maybe the fact that it just tastes good.
Yet the one night when he was throwing up ALL night long and ended up in the ER, he kept racing to the door to go outside and he would start eating grass in order to make himself get up whatever was left.
But oddly enough, I can tell a difference between his "I am just eating grass for fun" or his "I am sick and need to eat grass" haha. It's very rarely because he's feeling sick. And he never pukes from just eating grass on a daily basis.