It all depends on the specific people involved, but generally speaking I have found out that any food on my table in the rehearsal hall becomes fair game if it's more than enough for one person (i.e., a box of triscuits or animal crackers, not just one sandwich). When in doubt and you want it for yourself, don't display it. I actually had one director who would constantly eye my table and I'd say, "It's paper. You can't eat THAT" for specific items he'd strangely start eyeing that were near where food HAD been at one point.
During tech, any food on the tech table is considered fair game for ALL designers/director/SMs/Etc front of house. Actors rarely even see it. If I don't want people to eat, I'll close the box or whatever, and place next to me in the seat or under the table. That said, I usually find something like the large containers of Walmart/Sam's Club animal crackers that are cheap and relatively enjoyed by everyone that I will place to stave off everyone's nibblies.
Erin