NO, NO, NO, NO, NO and to clarify NO.
The dealer is right in respect that one of the DVLA checks will show the vehicle has tax, if the dealer is being honest and has taxed it.
BUT and its a biggy you can be done for failing to display, that still carries a maximum 4 figure fine i believe. Traffic wardens can call the police to dish those out, ever seen a car parked that has what looks like a red parking ticket? I have seen that with my own eyes although not to my bike.