I do it myself, and ya sometimes shop guys don't do it so well. Sometimes the person u give ur skis to forgets to tell the tech guy just to wax them and they might tune your edges too which, in certain situaions, can be tragic.
As stated above, waxing your skis is cheaper. Also, u can change your wax if the snow is different or the temp is different where you are going. It gives you a better understanding of how skis work, and it got me into completely tuning my skis by myself. Now I have a much better undertanding of ski construction, which also translates to better ski purchases.