Well I can only think film and have a grey card but never use it, as for many shots Kodak grey is all around e.g. darker grass and shrubs, deep blue sky, pavement, grey walls, sun tanned hand and so on. So if for example I'm photographing blossom against a blue sky background I know the shot will be ok, but if it was bright white sky background, I would point the camera to a green shrub or grass, lock the exposure and then take the shot.