If you are growing in water culture you'll want oxygen as soon as the roots touch the water. Better yet just turn on your air pump when they are planted and just go from there. It doesn't take long for roots to find the water surface.
In soil, there's no need. But if you wanted to accelerate your growth you can add H202 to your watering to give the roots an extra boost of oxygen. Personally, i don't bother. I've only ever used H202 in Hydroponics using a drip system and rock wool slabs.