Obviously I'm not European but: We do a full alignment every time; granted our average leg length is about 8 hours and the fastest turn I've ever done is 45 minutes. Going from memory, the 744 can go no longer than 18 hours of flying time between full alignments.
Reasons we turn them off after the flight and usually do a full alignment prior to the next flight:
1. After a flight, we don't know how long the airplane will sit there between the time we leave and the time the next crew shows up (and the plane almost always stays powered up with APU or external). If there was a power interruption and the plane went into standby power while it was sitting there, the IRU's could overheat if there wasn't a mechanic or someone else around to notice (normally someone should be monitoring the aircraft all the time while the APU is on, of course). The airplane is rarely powered completely down between flights.
2. We have no real way of knowing how long the previous crew went before a full alignment. Fast alignment is still an option that can be used at the crew's discretion, but SOP is a full alignment before every flight. So it's possible that the aircraft might be close to it's 18 hour limit.....there's no way to know for sure without asking the previous crew, who we usually don't see.
3. True it doesn't matter very much given GPS, but IRU accuracy is tracked by the company at the end of each flight....we have to send IRU error out as part of the ACARS post flight report. Also much of Europen airspace is RNP 5 which is easy enough to maintain on IRU's alone, so it's nice to know you can maintain RNP even if GPS is lost for some strange reason. There is no such RNP requirement in the USA's domestic enroute airspace structure, to my knowledge, so it would make sense not to worry about it as much there.