As per previous posts, my wife insisted on replacing our old rotten wood
vanity top with a new one which I am making out of stained white oak
with multiple clear coats of West Epoxy on top and on the edges...

The question is whether I should seal the bottom (whether with epoxy or
polyurethane). However, I wonder whether sealing the bottom would be
more of a bad thing since if water does penetrate the top then it has
nowhere to breathe out of and since the bottom side doesn't/shouldn't
get wet maybe there is no advantage to sealing it.

Any recommendations?