What is the advantage on saving the money for the overlord when you have 9 drones, and not filling your supply first and then getting the overlord?
In my theory, in the first case you get to wait the saving time with 9 drones, and get to wait almost the same time with 10 drones in the second case. Second case should be better, since 10 drones mine faster than 9? So why does everybody go with the first case?
All this is assuming you go 12 pool or something like that, where you will get more drones after the overlord. 6- and 8-pools are obviously different.