...from which grey-scale value does grey becoming white in your judgement?
I honestly do not know. I usually do not "paint by numbers." It is more, as you suggested, a visual judgment during post processing . Depending of the time of day, weather conditions, angle of sun, etc. snow can be blindingly white (in which case something around 250-252 might be appropriate, with some specular highlights blown perhaps), or more or less gray. It is not that snow can not be presented as gray, it is more that the grayness should match the overall mood.
My comment about gray snow as my pet peeve comes more from seeing too many underexposed shots by people who think that modern, oh, so sophisticated cameras and their metering systems can not possibly go wrong (this isn't directed to the OP, btw) and let automatic exposure handle the snow. As most of us here know, that isn't the case, as any built-in meter is designed to deliver "proper" exposure under assumption that the subject is medium gray. Using a hand-held, incident light meter, gray card, or simply overexposing by one or two f/stops, usually results in a more agreeable snow.