I believe there's no hard rule and it's very system dependent. Especially for power cords: the same power cord can make little to no difference on a given component and produce a "waw!!" moment when connected to another component.
I make no secret that I use some components that can be considered extremely cheap / an insult to the audiophile world, some might say/ still they work great in their intended purpose. For example, the tiny Trends class D amp that powers the > 500Hz range in my (fully horn loaded) system. In my system, that amp is powered by an eternal linear PSU that costs almost as much as the amp itself; interestingly, using a good power cord on that external PSU was one of the biggest cable upgrade I ever made to my system. Width of stereo image got so much larger it felt like I inserted some kind of digital processing / effect. Said power cord is almost as expensive as the amp itself - because it's a very cheap amp - so the percentage ratio is ridiculous but the effect is well worth it. It's the result that counts. The same power cord on my preamp made the sound thick and sluggish, but another PC with a very different construction performed miracles there. And so on.
In any case, while it makes sense to treat the source first, ANY bottleneck in the system will be heard, no matter where it sits in the chain.