The general answer I leave to an experienced winter sower, but for blue poppies, ease the lid of now.

Blue poppies(Meconopsis) are native to the Himalaya, and adapted to constant cool weather and don't like heat, and especially humidity. Cool weather, full sun and low humidity: it's a tall order, which is why they are difficult to grow here. When you plant them out, try them in different places to see what they like for you.

I tend to take the tops off of mine when they 1) touch the top-Some of my containers have very shallow head room... or- 2) start to send out second set of leaves on top growth signaling it is time to transplant (for me)- or- 3) they show signs of "heat stroke" or "frying" or "steaming" by too much time spent in too hot of sun. The latter you generally learn from experience, sorry to say. I use less than the 3" of dirt as suggested- so I transplant sooner as a rule- I also hold out on removing the tops real soon as it is such a nice micro climate for all those "late-bloomer" seeds that may still decide to sprout- ...
Go a head and go topless as soon as you are comfortable- but keep the lid handy to re-use if hard frost or heavy rain come your way- It is easier to slip a cover back on than to run around with flats of containers of babies that still need a bit of protection and shelter- trying to find space to shelter them! Experience talking again.....
Also- when the tops do come off- be sure to keep a close eye on watering- they are still seedlings- even if they are really rugged ones- and they do not take well to drying out- or flooding...
Geez- one would think with all this "experience" that I never saw a plant from all of these babes- but I sure have seen many grow on- more that I can handle!!
Best of luck to you-
Julie