In general, there is no rule for lighting setup although most of the food photographers today tend to use main light behind the “subject”. Don’t forget, the most important thing food photographers need to do (before start shooting) is to decide what kind of “look” they’re going for.
There are many ways you can approach the subject and present it in a different “mood” by utilizing the light. Either you go for high or low key, every “look” needs to be planned ahead and of course accompanied with adequate propping, focus and composition. Once you decide how your photo should look like, the next question will be where to place your main light source? Beside actual position and angle, very crucial thing is to adjust intensity. Too much of it could cause really harsh shadows.
Many people new to food photography place their main light(s) in the front of the subject which can cause all sorts of problems … over exposure, intensive reflections and loss of 3D effect. When working with strobes, I tend to place my main light behind the subject and fill the front or sides with secondary light or in many cases with bouncers (white cardboard or mirrors). Remind you, the main strobe is almost always filtered with appropriate lightbox or a simple transparent material. Keep the power of this light at minimum because food doesn’t really require too much of it. You can achieve the same effect by using a simple external flash (I use Canon 430 EX) but again try to modify it or at least bounce it off the ceiling or wall. As mentioned so many times before, try to picture in your head how light actually approaches your food subject. Where you can expect to see shadows? Which side of your food plate will be lighten more than other? Is this the way you want to present your photo? In my opinion, playing with shadows in food photography is a challenge but when done properly, these photographs look much, much better compared to shadowless image.
Sample 1: Image taken with main light placed in the back of the subject
Sample 2: Light positioned above the subject
Sample 3: Main light positioned on the left side of the subject