This is a little late for tillmancan, but can benefit others reading after this posting.



Yes, IF the egg container material is PAPER based, but not the plastic foam, it is perfectly acceptable to plant the egg carton "cup" with your seedling BUT...



the spacing between plants in the "paper mache" type egg cartons is TOO CLOSE TOGETHER for proper plant spacing once in the garden.



So what I do is, when the seedlings are ready to plant in the garden, I use sissors to cut the "cups" of the egg carton APART, and then plant the cups WITH the contained seedlings at the spacing distance recommended on the seed packet.



IMPORTANT: For this to work, the egg carton cannot be plastic foam, but must be the paper mache type made from ground up paper. Usually you can identify this as the plastic or paper/cardboard material are clearly identifible by looking at them. Also usually, the paper type is a grey color, while the plastic ones are usually bright colors, and shiny surfaced.