RoyK:
Good working practice is to use a voltage rating of at least twice the expected voltage. More is better.
Working voltage is not the same as an "absolute maximum rating". Semiconductor datasheets often highlight the abs max ratings rather than recommended working conditions. Capacitor datasheets are more sensible. Choose a working voltage greater than the maximum actual in-circuit voltage it will be exposed to, factor of two is unnecessary. The device is spec'd to run at working voltage continuously. Higher voltages upto the "surge voltage" rating are tolerable for brief periods (but may lead to greater leakage current).