Don't wear it, just show it when you think it will get you a discount. Many places as you say offer discounts to crew and offering proof that you are crew is no offence in my mind. However wearing an ID is stretching it a bit. By wearing an ID on an airport you are immediately recognisable by the public as some sort of an 'official', if not only to ask you where the nearest toilet is
Anyway, that's my view.