WD-40 is fine as long as you remember that its a solvent and that it is not a lubricant, as everyone has said reusing ball bearings is penny pinching and generaly not worth the savings. Also you don't need to soak them, if you are in desparate need to cleaning and reusing your ball bearings just squirt some on a rag and wipe them off, soaking is a huge waste of money
The subject of ball bearing grades (and the similarly related topic of ceramic v steel) has been covered at great lengths and most have come to the consensus that while higher grade bearings (and ceramics) do in fact provide better performance but only for professionals to whom 1/10ths of a second actualy do matter and that for any recreational rider grade 25 is pretty much as high as its worth it to go.