I never go above 220,220,220 for whites.(sometimes even 205,205,205 when the scene is brightly lit in exteriors).
When the white material has a lot of reflection and/or refraction (glass) assigned, you especially need to be careful not to use high RGB's, because you risk to break the law of conservation of energy. Reflecting/refracting more light than you put in is physically impossible and will always look wrong.