I have and I do.
Using washers and screws as 'cable clips' is not a particularly brilliant idea in the first place (although I suppose there's nothing particularly wrong with it simply as a means of holding the cables in place during plastering), but using washers of a non-ideal material and then painting them to mitigate that deficiency seems like an even less brilliant idea. If one wants to use washers, then, as I said, stainless steel (or nylon) ones would seem much more sensible - and the screw obviously also needs to be fairly corrosion-resistant.