mshilarious said:
A little is OK, but don't go crazy. You will just cost yourself money, and the higher rated caps are bigger and might not fit. Also I read something about a cap needing to see a significant % of its rating, like 30% or something, to prevent premature aging. Or something like that, I forget.
When I first read that, I thought you were nuts, since that's the exact opposite of how I've usually seen then used, but you're right---actually, you're not paranoid enough.
Actually, the recommended range is as high as 70-80% of their rated value. However, in a lot of consumer electronics and computer electronics, I've seen 35v parts used for filtering 9-12VDC power, which comes awfully close to that 30% figure when you take the tolerances into account. One reason for the difference is that under higher than typical temperatures, the electrolyte would break down well below the rated voltage, so 30% of rated capacity ends up being a much higher percentage of the actual capacity in those applications....
Basically, in a circuit, over time, the voltage handling capacity of electrolytics degrades (reforms) to the working voltage. The excess voltage handling provided by choosing a capacitor with excess capacity thus slowly goes away over time. After some number of years, a relatively small surge above the typical voltage can cause the capacitor to fail. The bottom line is that electrolytic capacitors are somewhat disposable and typically have a maximum life expectancy of single digit years. If you have an electrolytic that lasts more than 10 years... maybe 15... it's a fluke.
The degradation, of course, is accelerated by heat, extreme voltage spikes, etc., and I believe that the effect can be reduced by periodically ramping the capacitor up (slowly) to a higher working voltage, though in practice, this is rarely done. It might also be reduced by running the part through an extended burn-in period at the rated voltage, but I'm not certain of that.
Related readings:
http://yarchive.net/electr/electrolytic_caps.html
http://www.electronicproducts.com/Showpage.asp?Filename=itw.may2006.html
