If it rolls off the top, you will see the spider. If it rolls off the bottom, you might rub a black widow into your asshole.
Good enough reason for me!
Also, dunno why it sticks with me, but 25 years ago, one of the mtb magazines had a column called "ask uncle knobby" where he would give life or bicycle advice. Someone once asked him which way the roll goes, and he said over the top, to prevent scraping your knuckles on the potentially rough wall surface. I've presented proper roll placement as a safety issue a number of times, citing knuckle injuries, and really enjoy the baffled looks I get. Especially if people really know me