IMO tanks should always mark in low level dungeons. Especially now, with roulette.

At low levels you don't have access to your full arsenal of hate holding skills. And you're often being paired with DPS in at least partial Darklight. And even synced that high level gear in low level dungeons can pull hate pretty easily if DPS decide to go after their own target.

If the tank is not marking, however, the onus is on the DPS to take a second and make sure they're attacking the correct mob. Look at the hate meters. Be aware. Be like Smokey the bear: "Only you can prevent party wipes."