Hello Jamie,
This advice primarily concerns power supply caps. If we have two power supply electrolytic caps in series, with the overall supply voltage being higher than the individual voltage ratings (say 400V B+ with two 250V caps in series), then uneven leakage current between the two capacitors can shift the ideal balance of having 200V on each cap to being very skewed.
Consequently, when we put a resistor across each capacitor, there is standing current across those resistors that is generally much larger than capacitor leakage, and that swamps out any differences that would appear.
When you move to interstage and parafeed caps, these are DC blocking caps, so putting resistors across them kind of defeats the purpose of having them in the first place, as some of the DC voltage that the caps are trying to block will appear at the outputs. On the other hand, the leakage current of polypropylene capacitors is very low, but not low enough to ignore. What you will sometimes see is a 10-100M resistor across each capacitor to maintain voltage balance across them and to minimize the voltage appearing at the output.
To say the least, implementing this can be somewhat trivial, or it can completely change the design of the circuit entirely. Buying higher voltage rated components generally looks a lot better after working through something like this.
-PB
