I like to try and use a small structured wiring can (14") in the wall and then modify the front cover to have an opening trough on the bottom. If you want to get real fancy you can even put rubber molding over the cut edge on the cover to protect the cables. With 1-2 gang wall plates they always get full so fast, the cables just dangle and you always end up pulling more cables then you expect when the client adds locations.
If you go with a bigger can you can even protect your cables from the sheetrock an painting crew inside the can, which is another bonus. Like Craig says too, you always want enough cable to spin the rack around and move it way from the wall.