I prefer to size to the small-small combination, so that the chain doesn't drag on the top pulley/cage plates. I know sometimes that ends up longer than the more accepted method of running the chain on the large-large without weaving through the rear derailleur plus two links, but my rational for having a slightly longer chain is if I ended up getting a bent or broken link, I can shorten the chain without issue. I've never broken a chain out on a ride, but I do have a portable chain tool in the bag just in case.