Yes its fine to switch to the egg cartons on day 18 since thats when they get settled into position for hatching. I have done that and its been fine.
Oldmitegator,
I did worry about that alot but it went fine. Maybe its more of a concern with the smaller eggs that sit lower in the carton? But to that end, I have successfully hatched speckled sussex,golden lakenveldors and other small eggs and had all of the smalls hatch.
I noticed that even when they pipped low they were fine, the just zipped all the way or most of the way around and then pushed it up.
I am not trying to tell anyone its fool proof though, since I am sure there are always variables. But everything that peopled have mentioned, I also worried about but all went well.
I would say though to make sure your humidity doesn't go out of whack and let the egg carton get damp or wet.Then the eggs will be sitting in a wet carton and thats bad.
Some people use the foam cartons too and I am sure thats fine. I just don't like to buy foam (the environment) .