Power Conditioner vs A/C cables

I believe that good power filters are very useful. Both to filter your AC, and to block digital noise from the digital/switching power supply components (computers, TVs, other). This means a mains filter AND a 2nd one, can be fairly simple, just for the digial /switching stuff.
Power cords i have tested many times and fail to find an value, unless you are replacing something deficient- which is not all that uncommon, but easily avoidable for $20

note on some comments above.  IMO the gold standard for a real, practical power conditioner is an industrial isolation transformer.  if oyu get one in the 2 kilowatt range, it will deliver plenty of transient power capability, will eliminate and DC component, filter out HF noise and reduce/eliminate ground noise.  Yes some series filtering is also helpful - i do it.
I would in fact argue against buying these form high end firms and get ones that meet medical requirements.  Just like I did.  The put another filter between your analog components and the dirty switch mode power supply stuff, bane of our AC lives.  Thes put noise "backward"to the grid and the rest of your house.