You don't necessarily need to market board your gear. That said, keep up with your weapon by ensuring you're doing your class quests, and post 30 if you haven't gotten a dungeon drop or quest drop you'll want to track down a nearby NPC. Your weapon controls your aggro generation. For most classes I'd say gear shouldn't be more than 10 levels below yourself; for a tank, I'd put that closer to 5 (accessories an exception because they're harder to get your hands on).

At the end of the day, there's going to be the occasional DPS that makes maintaining perfect aggro throughout an entire dungeon a pain (Especially when you first hit level 50 and people have literally twice your gear level). The black mage who starts off an encounter with a raging strikes'd Flare is going to have to just deal with the fact that you can't be expected to hold aggro through that though. Do the best you can for the rest of them.