[ahecht]

A) Don't use 6 wheels.

The friction equation says that on a flat surface, four wheels will have the same amount of traction as six. Each wheel will have a max static friction force equal to the coefficient of friction (which is the same for all wheels) times the downward force on the wheel (which is the robot weight divided by four). If the coefficient of friction is CoF and your robot weighs 130lb, for four wheels:
Total_Fsmax = 4[CoF * (130/4)] = 130*CoF

For 6 wheels:
Total_Fsmax = 4[CoF * (130/6)] = 130*CoF

B) Don't use grippy wheels on all six wheels.

If your robot turns about its center, there is very little sideways force on your middle wheels when turning. However, there is a great deal of side-force on outside wheels when turning, and your grippy rubber fighting that sideways force is causing your robot to rock. Put grippy wheels on the middle wheels and slick wheels [or even omni-wheels] on the outside four wheels. Even with four-wheeled robots, many teams will put grippy wheels on one set and slick wheels on the other set (with 190, if we want the robot to turn, we always put grips on the back wheels) to improve turning.

C) Don't make your wheels in a straight line.

Make your wheels slightly V-shaped, with the middle wheels lower than the outside wheels, or make the center wheels larger. Therefore, at any given time, only four wheels are on the ground, but you still have most of the advantages climbing stairs or other obstacles that six straight wheels would have (although you will rock forward and back quite a bit). Some of the Home Depot shopping carts work like this.