I rode road tires on 23mm inner width rims for a while. It really opened up the tires a lot.
Quite the gain in comfort (the pressure needed was much lower), but a 25mm might be pushing it.
I think it really depends on the tire construction. I went down two times because once on the angle, I reached the end of the thread - lost all grip in a instant. It happened with Panaracer Race A (which don't have a very large thread).
At the moment, I'm killing an old Schwalbe Ultremo at the rear, and there seem to be enough compound on the side to prevent the problem.
Tires manufacturers are often quite conservative in their recommandation - and there is a reason for that I suppose ;-)