I almost destroyed a Brooks saddle in a single wet ride. Chances are the that your body will properly shield the top of the saddle while you are riding it. If you are not riding it, then cover up the saddle. You have to moisturize the saddle with Proofhide, or else there will be a host of bad consequences, but if your rear tire sends a stream of water up to the underside of the saddle, then no amount of Proofhide will protect the saddle from that. I was on an event ride during a heavy and sustained rain shower, when I waterlogged the underside of a saddle. I got lucky, in that there was enough tension capability left in the adjustments to correct for the stretch my saddle took. This was a Brooks Imperial variant, so the only lasting scar of the whole wet experience is that my saddle as a peculiar curl up in the center where apparently my butt also curls up.
A fender will protect the underside of the saddle from stream of water. If you don't want to do a fender, then I would use a non leather saddle. Brooks has its Cambium line of rubber saddles that are impervious to weather. I have them on my two bikes that do not have fenders on them. I find the Cambium saddle to be a bit of a harsher ride than a leather saddle, but so far they are still on the two bikes. I can't comment on any other brands of saddles.