[colt527]

Well, this depends on the material of the wheel mainly. If you are using solid rubber wheels, then you only need to lower them a very very small distance, just to take pressure off the outer wheels. If you are using pneumatic tires, the amount that you need to lower them is greater because they will squish down under the weight of the robot. This is just a general rule, the actually distance depends greatly on the individual robot with its many factors, such as weight distribution about the robot, where the idler is located on the robot, etc.

There is also a sort of balance between current and controllability. If you lower the middle wheel a lot, then you will maximize current saving, but you may encounter problems such as your robot drifting from one side to another, or teetering back and forth (which can be annoying). But if you don't lower them enough and all 6 wheel are touching the ground but pressure is off a set of them, you will eat up current trying to turn but you will have more controllability. So, I guess its best to experiment with your design to come up with what your team is comfortable with.