Along with Ryan's advice, many teams use
Omni-wheels to make their front pair* of wheels slide laterally with ease. You will still have good front to back pushing power. Also, your resistance to getting pushed will still be good on the end of your robot where the Traction Wheels are located. You will lose resistance to being pushed sideways at omni-wheel end of your robot.
Teams have been using this technique to steer better since '98.
#: or back pair, depending on which end is carrying the most weight. You want the traction wheels carrying the heavier load.
Andy B.