Ebb & Flow Question??

So I know when you use rockwool in ebb & flow your not supposed to let the water touch the rockwool when you fill the tray. my first question is so it NEVER gets wet?
My second question is if I used rapid rooters instead of rockwool can/should I fill maybe 1/2 or to the top of them since they don't hold water as much as rockwool or should I fill the tray to just below the bottom of them as you do with rockwool?

I use either rockwool or peat pellets. Either way I put them in net baskets with hydroten. I flood the table with the empty baskets, then I add hydroton until the water is just over it. Then I put in the pellet/rockwool and fill the remainder with hydroton.
That way water will just touch your rockwool and wick into it. Your roots will grow down as they go. What you do not want to do is have the cube completely under water as that will water log them and drown the roots.