> I have seen comments online about capacitors needing replacement on iMac G5s,
> is that standard practice on those machines?

It's long been standard practice on the first two iMac G5 generations (PowerMac8,1 and PowerMac8,2) but not on the last generation (PowerMac12,1), which is the only one supported by MorphOS:

http://www.maccetera.com/macstuff/imac_g5.html

Due to age, the issue may increasingly affect the last generation as well. I'm not sure whether Apple switched from electrolytic capacitors to ceramic capacitors for that generation. There are also aging effects with ceramic capacitors, albeit to a lesser extent.