It would be next to impossible to polish a crank shaft down that much and keep it round and with out any taper. It's Probably been ground down to a .0010 under-size on the main journals and .0010 under-size on the rod journals. This is considered to be the first under-size grind. the next would be .0020 then .0030 and so on. If done properly their shouldn't be anything to be concerned about. In doing this only .0005 metal is removed. It would be wise to get it checked for size, taper, run-out and checked for cracks since steel cranks are prone to crack.