step up/down power supply using mc34167
Don't underestimate the challenge of what you're attempting to do here. A variable-output power supply isn't trivial, as you now have to consider maximum and minimum load at multiple output voltages.

Also, your non-inverting buck-boost converter will be a negative impedance (constant-power) load on your source power supply - if the source supply output voltage decreases, the current being drawn by the buck-boost will increase. This can have implications for the stability of your source supply. 

All that being said, as to your question, there are always two considerations for the output capacitor of a switching power supply:


Voltage rating
Ripple current rating


The expected maximum sustained DC voltage on the output should not exceed 70% of the capacitor's voltage rating, for reliability purposes.

Similar voltage derating should also apply to the input capacitor. Fixturing is going to play a role as to how much work your Vin cap will be doing, as well as the number and nature of caps on the output of your source supply.

You'll need to evaluate your power train and determine under what input and output combination you will see the highest peak-to-peak inductor current, which will dictate the peak-to-peak output ripple voltage and power dissipation of the capacitor via its ESR. You should also derate the ripple capacitor current to 70% of its maximum - more margin is better, as cooler capacitors last longer than hot ones.

(IPC-9592A is a good standard to get an idea of component derating guidelines, among other things, for high reliability power supplies.)

Further to your comment; what will your output voltage be when your pot is adjusted to zero? I don't think it'll be within your target range.