[DonRotolo]

The decision to drop or not depends upon the traction of your outer 4 wheels. Less traction means less need for drop, more traction means more. Thus mikelowry's comment "Assuming you are using 6 traction wheels, you need to drop the center wheel. The robot wont turn if you don't."

Make a set of side plates that have an adjustment for center drop, get a bunch of different wheels and experiment. Best way to learn.