I follow Ed's line of logic. I never pot prior to seeing little roots formed, and will not transfer until either a) I have 2-3 a least 1/4" long or b) I have one single root which is exceeding 1/2"-3/4" in length, so that just taking it out of the bag would risk breaking it off.
If you are not already aware, new roots break very easily and should be treated as if they're made of glass. Once they get longer than 1/2", they become quite easy to break off.