If I know I'm going to have to eat during a production meeting, I like to bring sushi. It's already in bite size pieces; I can discretely pop a piece in my mouth when the conversation has turned to something I won't need to contribute to ("which fabric do you like for Sally's dress?"), it's not messy and it is quiet to eat.
I don't eat during a performance. Either 1) I'm backstage and there is no food backstage or 2) I'm calling the show and it is so annoying to be on the other end of headset hearing someone smacking away at their food.
I try not to eat during rehearsal, saving it for break time. Occassionally I'll work on a show that I just never get a break/time to grab a quick snack, so I'll try to discretely snack on some almonds, grapes, soft granola bar (small, fairly quiet). Again, I try to pick my moments so as not to be distracting or noticable to the actors/director.