Edit: Looks like I'm talking rubbish again...
I do not believe that using a capacitor rated very much above the operating voltage is in any way beneficial. For electrolytic capacitors it could even be detrimental as the rated voltage is gradually reduced to the operating voltage as the dielectric layer is not being maintained at the rated voltage.
So, simply choose whatever voltage rating is next up (in the make of capacitor you choose) from the operating voltage, and remember to include possible voltage peaks from ripple etc. in the operating voltage. Providing the rated voltage is not exceeded, you are good to go.
Also ensure not to exceed other rated parameters, such as maximum ripple current, temperature, etc.