Should countries advertise on their currency?

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Advertising is begining to invade societies private sphere and has been for a while, just think of the introduction of tv and internet vs. 1900. Yet money, which has a higher visibility rate than any billboard or television is not advertised on. Is this ethical or legal? If we imagine that such advertising would you support the decision if your country decided to advertise on their currency? I woudl imagine such a placement would be valuable and make a lot of money.