I've done some food and product photography, and very little of it is "straight" unaltered imagery. Shiny stuff in particular is a real PITA, as it's very difficult at times to find an angle that doesnt clearly show the camera or one of your lights in a reflection. So you try to shoot with as little extraneous reflection as possible, and then clean up in PS. Hi-resolution renders of shiny things can save huge amounts of time trying to shoot around reflections and subsequent PS cleanup. It's a practice taking hold in the auto industry, and some photographers specializing in that are losing business. It's cheaper to hire somebody to make a photorealistic 3-D model of the car (which is usually designed in a CAD program whose files can be exported to the 3-D modeling program anyway) than to pay a photographer and a couple of assistants to spend several days to set up a bunch of carefully-tweaked lighting in a specially-constructed studio and shoot the car, then another few days to do the PS cleanup.
Menu shots of food may be composited to keep one item from casting a shadow on another, or to avoid the "embarrassing reflection" problem, and there are all kinds of tricks, like freezing cakes and pies so the slices have neat edges, and salads and the like are carefully arranged and misted with water spray to look "fresher". And yes, typically numerous items are made, and the best is chosen to be photographed. Any shot involving ice cubes or chips is typically composited, as the layout of the items arranged in the ice is often subject to change, and rearranging layers in PS is much easier that reshooting the whole thing.
In general, advertising and marketing images have even more airbrushing, cloning, compositing, and other fakery in them than fashion and glamour images.
