I assume your speaking of replacing the balls in a conventional cup and cone bearing. (notice the bearing is the entire assembly that supports the hub on the axle).

I prefer to replace the balls whenever I overhaul hubs or other bearings. It isn't usually necessary, but it's small dollars for peace of mind, and saves me having to worry about dropping one and having to find it.

Grade 25 balls are relatively inexpensive and better than what you actually need, so indulge and go first class. There are yet more precise balls, but grade 25 is what's used in electric motor bearings, so figure that if they can run at 3600rpm thousands of hours they'll serve fine for your bike.