I have kept mine in my wallet for 7 years now after one was nicked, never been asked for it. Even when i Had a off and the police came to the accident they asked about my tax and i said 'yeah in my wallet' and showed them they were happy.
I also thought now with all the computers etc the traffic wardens can radio in ur number plate to see if u paid even if ur not showing it?
Theres no where gd to put one on a bike either