I think @orrpeople
and I have been reading the same sites. I believe they said not to use egg cartons because you need more air circulation; however, cutting out the bottom provides that, and then it's fine to use them. That's what I read, for what it's worth.
What I do is cut circular slices out of a paper towel tube. That works beautifully to prop up big eggs. For smaller eggs, I just cut the paper tube slices into a smaller diameter and tape them. It's worked well, and I got beautiful, healthy chicks even with huge, saddle shaped cells.
Yes, I was expressly told to use them with shipped eggs, then gently tilt one side up at a time to turn them - I also use an exacto to cut holes in the bottoms and to pare the sides down a bit too. I have a feeling this is all a bit like parenting advice. It can make you a little crazy!!