I can only talk for London. There are no permits or parking costs, so no worry over where to stick them or keep them secure. Having said that, there are now many times more bikes than parking spaces, especially following the introduction of the congestion charge (from which bikes are, also, excempt). Hence the increasing number of bikes parked on the pavement with reg plates covered by a plastic bag: traffic wardens aren't allowed to touch the offending bike and therefore cannot issue a ticket.
Back to the substantive point: tax discs are usually secured as part of the (lockable) hinged seat. And parking tickets, when issued, usually get tucked under the strap most bikes have across the seat. No-one bothers to steal them...