Generally the reason not to put knives in the dishwasher is because the mild abrasives used in some detergents, and more importantly the agitation which bonks them around, can dull the edge (and damage the coating on the racks that prevents your dishwasher from rusting, if you have one that isn't full plastic) (also not promptly drying the knife will lead to rusting if you have carbon steel instead of cres knives).