Yes, melee move back and forth constantly to maximize damage. As for leveling, it depends how you level. If you level through quests, it doesn't matter because nothing is tuned so tight that a front-only DPS wouldn't get through them, quite often you also have quest NPCs tanking. For FATEs you'll usually have someone tank also, as it's best to go around in a small group. If you're planning to level through dungeons, it's best if you group up with a tank for the queues since DPS have the longest queue times. That will slow you down beyond no control if you're alone but it doesn't have anything to do with class mechanics. For the fights you'll naturally have someone else as tank.

Melee classes make up the DPS loss of disconnects by doing more damage when they're connected, and during most fights you can stay on the mobs almost full time when you learn how you need to move.