Never. I'd prefer it if more checks were carried out though - I know for a fact most of the anti-social anglers out there don't have one, they'll even brag to you about it! I know baliffs have the right to fine you on the spot, however I think this should be handled more like the police where you are expected to present it with in 14 days. Honest people make simple mistakes and get stung, freeloaders seem to get away with it continually.
Never had mine checked. I pay for various licences/day tickets and always get one without fail, have never been asked to show one, but I know the day I forget/dont get one I would get stopped. I know people who constantly brag about not paying licences/day tickets and never get stopped, it really f**** me off they dont pay, but you have to pay to help with the upkeep/runnning of the various waters, cant have owt for nowt.