Like others say, they are fine unless you get them wet, in which case you have more of a problem with the oak than the magnets probably. Even then the plating on them should keep any staining problems at bay. The easily available type (shiny ones) are nickel plated quite heavily as the material itself is very delicate, like eggshells, the industrial versions like those made into motors are probably not suitable but then you'd have paid £50+ each and would be unlikely to be using them in the first place
Aidan