It all depends on YOUR schedule and how backed up you become. I agree the less you handle the young cells the better. I have been successful a number of ways. By : 1) removing newly drawn (uncapped) cells to a finisher colony. 2) removing freshly capped cells to a finisher or to a incubator. 3) leaving the cells in the starter colony. 4) Installing freshly capped cells to the mating nuc (not my preference but you got to do what your schedule permits). As far as introduction to the mating nucs, alot depends on the time of the year. In and near the “Great Smokies”, we can have some very cool nights (not the typical southern weather). Therefore, early in the season I try to hold the queens till day 15 (day 11 after graft), later in the year I will introduce the cells a couple of days earlier. Again it all depends on the weather, population of the hive etc.
There is no straight answer so don’t chastise someone who states “as long as possible”. Sometimes it’s good to experiment or get a little ink on your nose doing some research. That’s a perfectly good answer in my opinion. I also employ a “rack” that confines any early hatches to themselves in a hair curler type cage.
That said if I have a choice, I would introduce the cells on day 15 (day 11 after graft). Out of 30 or so queens I invariably have 1 or 2 virgins to hatch and introduce.
I took out a graft today that I grafted last tuesday, 10 days ago. Of the 24 started cells, I had about 6 with holes in the side. Sure enough I found a virgin and put her in a cage that I carry in my pocket. I thought I was early pulling the cells today but I may need to start pulling on day 9, especially if I will lose 25% of the cells due to early hatching. Of course maybe I grafted a larvae that was a bit too old.
We pull them on day 9 also. Maybe it's the heat and humidity in FL but I lost 2 batches of 45 cells each last year due to early emerging virgins. Not a good deal when you have already made up all your splits and they're waiting very impatiently for a queen. Yikes! Scramble time!