I prefer using 1/2" stock for axle shafts, having cut my FIRST teeth in the days before mandatory bumpers, when open wheeled robot to robot contact was the norm.
For '12, we went with an 8wd chassis with a live axle arrangement on our front and back wheels. Axle shafts are supported by a pair of R6-5ZZ’s in a bearing block, cantilevered west coast style and have run without issue. We have used these bearing successfully now since the '10 season.
They can used in West Coast Products Colson Hub or AndyMark’s 375 Bearing Hub (am-0161) to upgrade to 1/2" round shafting for dead axle systems. They are lighter and can be packaged in a smaller envelop than the standard R8 series bearings.
Half the battle during FRC build season is finding where to buy the stuff you need or want to use.