When to Transplant into Rockwool?

Hi all! Could anyone guide me on when to put my newly hatched seedlings into the rockwool cubes? The taproots are already trailing out of the little propogation cubes, even though only the cotelydeons and the first two tiny jagged leaves are showing on the seedlings. My instinct is to put them into the cubes now because the taproots are out of the bottom of the propogation cubes....anyone?

 

Is done already I couldn't wait I'm such an impatient soul!