If you have plans for a spinner, I'd recommend buying the extra bearing support that bolts to the front of the gearbox, or better still having a bearing support on the other side of the wheel(s). By nature, spinners see huge forces, as not only do they receive the same blows as other bots, they dish them out!

If you mount the wheels very close to the gearbox face you may not need this extra measure, but it's all in the aim of gaining reliability.