I can't really qoute you a precise source, mostly just my own experience.
The quicker you plant the seed (once the tap root is visable) the better! You see, the thing is, the longer the taproot gets; the more it is prone to getting damaged. The taproot is a fragile thing and it does much better when protected in the soil or Hydro system (I'm not a Hydro guy, so I take the later for granted - someone correct me if I'm wrong).