[Matt Adams]

Some quick thoughts on your concerns...

In general, it's a good idea to avoid more than two bearing on a single spinning shaft, and it's best to have a bearing on either side of your power transmissing device. (gear, sprocket, etc) Since the shaft is presumeably already in line, more than two ball bearings tends to just add friction... there are naturally some exceptions.

You're always going to want to support both ends of a shaft if there's going to be any sizeable force applied to it.. such as wheels. Don't ever have wheel's cantilevered off the edge of your robot! As for a motor, since its only function is to spin about its axis, it has no other forces acting on it besides the force trying to kick the gears apart. Since the length of output shaft on motors is so small, you can typically get away without supporting both ends, which is why in many of the designs you've seen they don't.

Hope this helps,

Matt