I’m not sure there’s any reason to think that a spring using modern materials and manufacturing and one that’s without other defect will loose tension over time provided it’s used within design tolerances. I think concerns over springs in ski bindings date from earlier years when more general purpose components were used but I’m guessing, it could just as well be that it was never true, springs have been around since 1300BC so engineers do know how they work.
Some skiers loosen their bindings religiously each year, partly as a habit I guess but partly because it’s a received wisdom and despite having no real basis in fact it probably reassures them they’re “in the know” or something.
I’ve yet to meet anyone who’s actually had a binding fail due to a spring failure and I don’t know many people who loosen their springs.
But, old bindings can have a load of faults, in the US there’s an “indemnified list” which has a legal basis, if you break your leg on a binding the manufacturer said was too old then they’re not going to consider themselves liable. Here in Switzerland you can get a little MOT type sticker when your gear is serviced which is much the same I guess. You can check your bindings against that list : http://www.nssra.com/2001/nssra/index.asp (it’s the 2010 list, ignore the 2001 in the URL)