Where to use aftermarket power cords?

I think a reasoned approach would be to purchase a cord of your choice and try it in your system in each of the components you mention. The impressions you get will be based on your experience with your system....not anyone elses.

The previous answer is correct as far as it goes, but I don't think that's what you're looking for right now. When I replaced my PCs I started at the source. I enjoyed the improvement so much I continued all the way back to the amp.

I tend to think the maximum improvement will be recognized by starting at the source.

In my opinion, they're equally important. It really depends on how the power supply design in each unit.