Our downstairs (living areas) are fully tiled, whereas upstairs (bedrooms) is fully carpeted, so for us that's the difference and the line is the bottom of the stairs. One exception is the kids may not eat *on* the couches in the living room (I can, they can't!). However as it's open plan the living room/dining is technically one room, so they definitely eat in that room.
I prefer the kids not eat in the downstairs playroom as it's annoying to clean, but I do make exceptions for a "special treat" as my 3yr old calls it. Basically, if I am desperately in need of some kid-free time and am treating them to some TV/DVD, I figure they may as well have a snack in there too.
I strongly encourage my kids to eat outside on the back porch, easier to hose them down afterwards... (I have a 1yr old and 3yr old and live in a hot climate!
Bedrooms... perhaps if they weren't carpeted? But it's nice to have part of the house that stays that little bit cleaner, no crumbs around! No ants.