Some years ago, I took the added precaution of adding a nitrile o-ring to the seatpost above the seat collar. After generously greasing the seatpost, there was usually a little rim or meniscus of grease left when the seatpost was installed at my riding height. I simply pushed the o-ring down into that grease, against the top of the clamp.
I do think it makes a little extra positive difference in shooing water away from the upper post/tube interface, though often the real culprit in this area can be the stress-relief hole at the bottom of the saw kerf or slot in the top of the seat tube. This is where the most distortion can occur under clamping stress, and it doesn't take much to create a little channel to guide tire-thrown rainwater right inside.
This is where generous pre-greasing comes in to form a barrier to water entry. If you choose (or happen!) to ride in the rain consistently without mudguards, then rigging some sort of small shield or deflector over the clamping slot also helps. I once made one for a friend using a small piece of sheet aluminum that was held in place by the seatpost bolt; it made a little shed roof that covered the slot from the rear. I also used to carry a tire sock (an underseat bag shaped like a baguette and intended to hold a spare sewup tubular tire or folding clincher below the saddle rails and it secured to the stays below the seat cluster) and would often hand this off to 'guardless friends when riding in the wet (I used mudguards). It did a world of good for keeping the inside of their frame dry, though it always looked the worse for wear afterward when they handed it back!
Best,
Dan.
