When tanking I usually mark all but 1 mob on a pull (unmarked defaults to last). I just find it keeps things orderly for the DPS. I've played a lot more DPS than tanking and I always much preferred marked targets, so that's what I do when I tank. As a tank I've noticed that if I don't mark something people just attack whatever. Sometimes I'd see the DPS change targets because they saw me using Butcher's Block or similar on other targets to get an enmity lead and thought they were attacking the wrong enemy. I've even seen instances where we're all attacking different things and keep switching targets at the same time to try to all attack the same thing, but fail. Kinda funny, really.

Basically marking never hurts as long as you do it quickly and sensibly (as in prioritize the mobs that have annoying aoe or do the most damage). At it's simplest you can macro 1 onto tomahawk/ shield lob to always mark your pulled target as one and then mark during battle as needed.