You want your roots as dry as possible without hurting the plant. The plant loves to get oxygen through it's root system, and it's harder to do if the roots are covered in liquid. The best case scenario would be to keep lowering the nute level as the roots grow longer, but keep wetting the roots intermitently so they don't ever dry out completely. To protect from power outages, I aim to have a least a couple of inches of roots in the solution at all times. Otherwise I would aim to keep none of my roots in the solution. I have a way to vary the level of nutes as the plants grow, but I keep spraying the entire root system about every 10 minutes.

I hope that gives you some ideas.