on a well-regulated 5V supply, a 6.3V electrolytic capacitor would be fine
that's a way how Chinese guys doing it. In order to make electronics more cheap and less reliable. They don't care about reliability and durability. Because if it will fail, they will get more money. For 5V working voltage, 6.3V capacitor is 25% voltage margin - this is the minimum voltage margin for capacitors. And this is why Chinese electronics fails very often and don't works for a long time.
In opposite, when you're need reliability and durability, such as military electronics or something like that, there is need at least 30-40% voltage margin for capacitors.
The cheap Chinese crap, has crappy capacitors, which don't meed their specifications and have a high ESR, so will fail quickly. Decent capacitor brands are far more reliable.
De-rating also depends on the capacitor. Tantalum capacitors definitely need de-rating, aluminium not so much so. Running at too lower voltage i.e. too little DC bias isn't good either, as some voltage is good to keep the capacitor's oxide layer in good condition. I wouldn't recommend a 100V capacitor, in an application rated to only 10V.
It's simply not true that using a 6.3V aluminium capacitor on a 5V supply, compromises reliability. Capacitor size roughly quadruples, every time the voltage is doubled (E =
1/
2CV
2), along with progressively having a poorer ESR, if the package size isn't increased enough. A 10% to 25% safety margin is fine, giving a roughly 20% go 56% oversize, but going far beyond that, it starts to get silly (40& higher voltage rating is nearly double the size!), especially if the power supply voltage is well-regulated and it's not being operated at high temperatures. It makes perfect sense to use a smaller, lower ESR capacitor, than a much larger one, which will quite likely have a higher ESR.
As far as we're aware, this is a home audio amplifier, not a military grade piece of equipment! Even in designs for aerospace and defence, there's no blanket rule on de-rating capacitors. It depends on a number of factors such as: how long it will be in service for, before it will need to be replaced, the temperature, size, weight and space restraints. Using much larger capacitors than necessary, might be a problem in an aerospace design, which needs to be as light as possible. I used to work for a defence contractor and we had a reliability engineer who would look into the mean time before failure of different components in a system. Electrolytic capacitors are nowhere near as unreliable, as many believe them to be.
In this case a 35V rated capacitor on a 32V rail, might be OK, if that's the nominal voltage and it doesn't get higher than that, or it could be marginal, if the power supply is poorly regulated. It also depends on whether the capacitor is subject to high ripple current or is hot i.e. next to a heat-sink. Of course, if the capacitor needs replacing and you can only get higher voltage ones, in the same value and footprint, then go for it, otherwise don't bother.