It's an interesting idea, soaking Brooks saddles, but sounds a bit drastic. I'd give it the proofide treatment first and only soak if that fails.
I've a saddle over 20 years old and another that's 17 both in great condition with just proofiding. Not very often, either. I proofide only the top and just leave it for a few hours but I don't see why you shouldn't proofide both sides if you want.
People are fearful about the saddle getting wet but I guess they get pretty damp with sweat over a long ride.