Unless you actually work on an assembly line, there's no excuse for assembling sandwiches or other meals at your desk.
You make your kids pack their school lunches the night before, right? There's a reason. No one needs to watch (or smell) you smear mustard on naked bread and then pile on the deli meat just so.
Worse yet, they might ask you if you have any extra, and do you really want to share your extra lean turkey pastrami with Bob from accounting?