Frankly, if I was showing a vehicle and had an out of date tax disc in the window I'd be grateful for a member of public for pointing out to me that I'd forgotten to change it for the new one, before Plod fines us for not doing so...
Can't see the point of fake "old" tax discs anyway, it's a small circle in the corner of the window. Can only understand it if it's a real one from the vehicle's past.
I wouldn't care enough to report, unless it was clearly dodgy (e.g. a ringer) at a show, but would if I saw a vehicle on the road. If you can't be bothered to tax it then I suspect you may not be bothered about insurance either, and given that us honest folk pay through the nose for the privelidge of being legal I'm not too happy about that.