Electrolytics are generally chosen for the next voltage rating higher than operating. A 12V supply using 16, maybe 25V caps; a rectified 120VAC line (= ~160VDC) using 200V caps; etc.
It's not generally recommended to use vastly higher ratings (e.g., 100V rating for 25VDC) because the capacitor degrades over time, resulting in off-spec value, ESR and such.
Electrolytics degrade over time because the dielectric very slowly breaks down in the electrolyte; effectively, its voltage rating decays over time (years to decades). This process is reversed by leakage current, which causes an electrochemical reaction that reforms the dielectric. After enough time, the effective voltage rating drops to the voltage it's being operated at. If this voltage is a lot lower than normal, the thickness of the dielectric, electrolyte and electrodes will all be different from spec, giving different C and ESR values, probably out of spec.
In contrast, you generally choose tantalum capacitors for several times nominal operating voltage, for safety reasons; the same goes for ceramic caps, but for stability reasons; aluminum polymer and film capacitors are fine at ratings, and also generally expensive or bulky enough that you don't want to get vastly overrated parts anyway.
Tim