As far as I recall any punting venue that I've ever been in with a prominent clock has always been set to UK +10mins - isn't that a punting given a bit like the stated-age +10 years rule?
The start of every punt I ever have follows the same model: I shower first THEN I hand over the loot. As I hand over the loot, I re-state the services that I requested, the duration that I booked and finally I say "it's ten past four" or whatever, just to let her know I'm "aware". I've only been short-timed by a Tom a handful of times in fifteen years (I've legged it early after draining my spuds quite a few times but that's on my terms, not hers).
This is a financial transaction after all - money in exchange for time/services.
The only scenarios that mess with my system are knocking shops, where there's no clocks anywhere, the Tom disappears with the loot for 10 mins before we get going and the maid invariably gives the 5 min knock at least 5, sometimes even 10 mins early, so I tend to avoid them (unless there's someone I REALLY want to see there)