the tyres dont need stripping off the wheel,all they do is break the bead on the bead breaking machine (hydrolic arm) they then pull the valve out and put a new one in and pull it so it sits in place, this should take approx 1-2 mins per wheel its very simple and quick ! a valve is worth about £2. you should look to pay £5 each, most tyre shops will do all 4 for a £10 drink money - not going in the till