No, you don't have to cook for them. but it sure is a nice gesture to atleast offer.
"Hey man, you hungry? I'm making sp'gehtti."
"Awww, yeeahh, hellz yeah dude thx!"
"No problem, can you spot me like 2 bones for materials? I'll still make it all" (you were going to make it and clean it anyway)
"Yeah man, no problem."
"Sweet, thx"
"yeah"
"cool"
If your roomies are people you don't really know, this would be a good way to get to know them and have a more casual/pleasant atmosphere in the apt/house. They're sure to reciprocate (unless they're real d!cks). I speak from years of roomating experience. Sharing is key.