I tend not to over rate capacitors too much.
Higher voltage capacitors are larger and also have a higher ESR and ESL.
Quite the opposite - higher voltage capacitors are normally lower ESR - which is why it's advantageous to replace faulty ones with higher voltages, if space allows.
Where this falls down though (drastically), is 'very' high voltage ones, commonly used as start-up capacitors in SMPSU's. These are usually between 1uF and 10uF rated at 450V or so - these, even when brand new, have particularly high ESR - but this is no problem for their useage.
But to go back to the original question, the capacitors used in my tutorial - they are all low values, so are physically small, and certainly the 1uF's I used were going to be rated at 100V, lower ones aren't any smaller, or any cheaper - so that's the only 1uF I stock.
Once you get to larger values, I keep multiple voltages - so for 470uF I keep 16V, 25V, and 63V - not so much for the voltages as the size, if I could get 63V working in the same size as 16V, I'd just stock 63V ones, but you can't
I don't generally stock anything below 16V, as it's too restricting.