If you're planning to eventually get all the packs on each account I would take IoD on each because most expensive. The caveat is that the IoD bundles are worth the asking price, since the top end includes two mounts, a great hireling at heroic and epic levels and the free boxes. Obviously it is extremely expensive to do that across 4 or 5 accounts.
If you're only planning to get all on your main account eventually I would take a different x-pac on each account, starting with IoD for the first if you are not going to buy it, and plan to have them buddy your main 1 on 1 through the leveling process.
My experience with more than dual-boxing in DDO is dismal. And I'm pretty lucky normally with loot pulls. The problem is playing more than 2 characters at a time is a *very* slow process and speeding it up essentially involves abstaining from taking loot on the alt accounts and even then it really slows you down.
Your mileage may vary depending on if you have some way of controlling multiple characters in real time.