Do you do the same thing day in and day out, or do you mix it up because you're bored? Now tack 1,000 years onto your life and answer the boredom part again.
Low end green dragons slide the target 2 squares, which is not always going to be enough to throw someone over a ledge. Even when they do, there's a 55% or better chance that the target falls prone instead. Higher level dragons can go up to 4 squares, but are then contending with character who have better defenses against falling.
It's a good tactic. It's not always the best one, nor is it always available. Sometimes you'd rather put another character inside your breath weapon's area, especially since it could mean more damage (keeping in mind the DMG fall guidelines). Pulling them closer to your frightening presence could also be fun.