I'd say that if you're using galvanised plate then use galvanised washers, there shouldn't be any corrosion as a result, but dissimilar metals in contact such as stainless and mild steel will cause corrosion in the presence of water, particularly salt water
As an aside I was just wondering what people have done re tilt alarms. I assume there's not one built into the standard alarm already fitted? Any recommendations for a supplier?