While the power cord is part of an overall electrical circuit, there is a basis for the "first six feet" perspective. This is mostly because of the greater potential for noise in the environment from the wall to the audio component, than inside of the wall.
The primary culprits seem to be:
- induced noise from electromagnetic fields interfering with nearby (signal-carrying) audio interconnects,
- high-frequency noise (RFI) from Wi-Fi, cell phones, rheostats, and appliances that is more prevalent in an open room than inside the wall, and
- feedback in the power line resulting from other audio components, and particularly those with switching power supplies.
Therefore, whether considered the first or last six feet, the component power cord is more likely to pick-up and/or transmit noise to an audio component, or system, than the in-wall wiring preceding it. However, assuming the power cord has adequate wire gauge for the current being transmitted, and is properly grounded, the ability to resist noise seems to be mostly related to having adequate shielding, and possibly noise reducing geometry, than about fancy metallurgy and other attributes that drive stratospheric pricing. In addition, the quality of the component power supply also plays a big role in overall system performance by adequately filtering and regulating the incoming AC into clean, stable DC voltage as demanded by the component.