Do PIV's add?

[Mike Watterson]

In most cases (of diodes and capacitors), resistors aren't needed.

For selenium rectifiers it's true.
Capacitors need derated by a factor related to tolerance spread. Electrolytics especially even for only two, need resistors as leakage can be variable. Capacitors and silicon rectifiers ought to have high value resistors, especially if more than two parts are in series.

It depends on the level of risk / reliability. In the 1970s Hawker Harrier head up displays were failing because the multiple ceramic discs in the EHT capacitor package were not matched enough, nor derated in working volts due to tolerance spread.