Beginner Question: Air pump?

[malangon]

I am brand new to hydroponics and I'm just starting to experiment. I built 2 "raintowers" which are essentially 6 foot tall PVC structures which rain/drip the solution down on the inside.

My question is: In doing some further research, I wanted to add something in addition to my raintowers (which I haven't fully implemented yet).

My idea was to construct a system using horizontal 4 inch PVC with net pots and a system with a tank that runs water through the PVC pipes. Forgive this basic question, but two issues confuse me which I can't seem to get a straight answer:

1) Should the roots stay submerged with the water constantly flowing or do I put this on a timer and do a "ebb and flow" type setup? Why one way or the other?

2) I have seen systems with an air pump and air stone. I get it that O2 is important, but is it important enough to to to incorporate an air pump or is that an optional item?

Thanks!