From a list discussion a couple of years ago ISTR that there
were two points to consider when selecting either foil or Ta
caps

For foil caps, choose the voltage rating that is nearest (and
obviously above) your voltage rail. A 200V-rated cap on a
5V supply may not re-polarise if the circuit is not used for
some time

For Ta caps, allow a generous over-voltage margin (50%) as
they are wont to fail if pushed anywhere near their voltage rating.
For example use a 10V on a 5V supply, 25V on a 12V supply etc

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