This is likely to be a "can of worms" thread. I am sure that there will be many who will read this and say that they know better because they put a line conditioner on their system and immediately saw a big difference. For those people I would suggest you not read further. I do not intend to deny nor debate your experience. I simply relate what I have found in trying to identify what is going on with power supplies and noise, using a variety of equipment and a decent oscilloscope. For those that want to better understand the technology, read on.
The topic of power conditioning came up in the Surge Suppressor thread and I did not want to go off topic there, so I'll discuss the matter here. The question arose in a discussion of surge suppressors for a projector. The thing that most people do not understand that is important to this discussion is that most video products use a switched mode power supply. What that means is that low level line noise almost never enters the signal path through the a.c. line. I'll explain why in a moment, but first let me give the rest of the story.
It is possible for line noise to enter a system, and for conditioning to be useful, but not the way most believe. If there is noise on the ground of a system, it can degrade video, but this is rare in a well designed and properly grounded system. Also, most A/V receivers still use conventional power supplies that can allow noise to pass that would never make it through a SMPS. This is also unlikely as most are well filtered and regulated with respect to video switching circuits, but it is more possible. Also, the SMPS in another component may itself create far more noise than present on the a.c. line. While it is unusual for this noise to actually affect other components, this too, is a possibility.
So why does a.c. line noise not get through a SMPS. Well, that requires an understanding of how they work. A switching power supply is designed to eliminate the massive transformers and large filter caps needed in a conventional supply. It takes the incoming a.c. and rectifies it to d.c. then filters it with similar filter capacitors to a conventional supply, only not on the same scale. At this point, because the a.c. was not passed through a transformer, it is actually more likely for noise on the line to have survived. Now the d.c. is switched on and off at a much higher frequency than a.c., anywhere from a few kHz to several hundred kHz. This switched d.c. then looks like a very noisy, spiky series of pulses. It is applied to a transformer (much smaller than a conventional p.s. transformer) primary. The secondary side of the transformer typically has multiple windings for various voltage levels and there are may be windings to control the saturation in the transformer for regulation purposes. Current provided is controlled by altering the frequency of the switching, or by varying the pulse width. There may be feedback from the secondary for control or the supply may be tuned to operate at a particular level. In the secondary of the SMPS, the pulses are rectified by a relatively high speed switching diode in most cases, then filtered with a cap and perhaps an inductor. There is usually some additional regulation on low voltage supplies that feed video processing, with additional filtering after the regulators.
Now lets consider some line noise. Generally, it is relatively low in level compared to the a.c. When it is rectified and filtered in the primary of the SMPS, some of it may still be present, however, though much of it will be filtered there as in a conventional supply. As the d.c. is switched through the transformer, however, the low level noise becomes insignificant compared to the switching pulses. These are typically in the same region as the noise itself (note the filter ranges on line conditioners, if they have those specs). After passing through the switching converter (switching transformer) and being rectified in the secondary, it is impossible to see any of the original line noise. It is far more likely to see remnants of the switching pulses themselves, as they would have orders of magnitude more current in the primary than the noise. After more filtering and regulation, however, most supplies to video processing are pretty clean.
I have looked carefully in many supplies for line noise. I have even generated noise and compared the regulated and unregulated outputs of SMPSs with and without line conditioning. I can easily see effects of filtering that reduce or eliminate line noise in many line conditioning products ON THE A.C. LINE. I have never, however, been able to identify that same noise in the secondary of a power supply. I have even looked at the garbage you get out of cheap UPSs that output essentially modified square waves. No evidence of it in the secondary of a SMPS, though some supplies don't run very efficiently on them and will even shut down.
So is line conditioning effective. I would say it is (in many high quality conditioners) very effective at reducing line noise on the a.c. line. Is it going to improve your picture? Not likely. It is possible, but not likely. Now conventional power supplies in most audio units are another matter. Even here, noise is hard to identify in the regulated and filtered supplies for most video processing. It is more likely to be present, though at very diminished levels, in unregulated amplifier supplies. IME, it is unlikely to find a meaningful difference in image or sound quality due to line conditioning. It is far more likley to be affected by psychoacoustic and psychovisual factors such as expectation bias as a result of marketing hype, sales pitches, and the related experiences of others.