You were right, the looser the ENO the flatter you can lay...but that's laying on the diagonal, not on the centerline. If you want to lay on the centerline of the hammock, then yes you will probably want to tighten the hang up some.
Take a look at this pic
. That's a good representation of what an ENO looks like when hung 'loosely', using a structural ridgeline of approx 100 inches.
Grizz is right about the ridgeline. If you use one, only pull the suspension tight enough to remove the sag from the ridgeline, not tight enough to make it overly taut.