[Jeffy]

If you can't find a way to make perfectly center drilled shafts, you are not alone.
Here is what we did:
We used 1/4" aluminum tubing with about .172" ID.
Then, put tape over the holes in the motor and connect it to a power source and turn down the brass gear to ~.175" ID.
Wen you go to press on the shaft, you need to be cautious to get it lined up straight. Also, you will need to support the end of the motor AND the gear. We used a pair of steel rulers and a vice for this.
A note on wheel size:
The optimal size is different for each minibot. You will need to know your weight and estimate the amount of drag you have correctly. If you need help with the math, first try your team mentors and physics teachers. If they arent helpful, I'm not sure what they are doing, but nonetheless, then CD can help.