This is a good one-I think of Wallies on a wall and Wallies off a ledge height obstacle as two different tricks. When wallieing something lower than waist height, you can center your torso and head over the obstacle as you jump it. This allows you to head more straight on. For something taller, you can't center yourself so you have to approach the obstacle more parallel, or at least thinking parallel in your head. You still want to get all four wheels on but your aim and body position is quite different (at least ime)