A drop between 1/8th and 1/10th of an inch will do you plenty of good, assuming you take a bit of time to see that your superstructure is remotely balanced. Otherwise, you might be better off to shift the center wheel underneath the center of mass for your robot (if you need to find this, CAD is your best bet).

Oh, also: Post pictures once you make it! We all like to ogle them shiny robots.