my flatmates leave the kitchen a pigsty and i can't stand it or them. i would say try to set the ground rules early on what matters to you, 'cause if you leave it for too long it's very hard to suddenly start asking for stuff, know what i'm sayin?
i've never had a problem with anyone stealing my food, but i also go shopping regularly and don't keep a lot of stuff in the kitchen at one time, so it's easy to keep track. i suspect tho that if you buy milk and condiments, it's not uncommon for flatmates to take stuff so i guess you have to be a bit flexible or hope you get decent flatmates.