How much dirt is too much dirt

I have no idea quantitatively. I have been pursuing high end audio for fifty years and each step with cleaner power has instant benefits to the sound quality. So, hence good quality preamps, DACS, streamers... etc, weigh 40 lbs or more, most dedicated to power management. Really good equipment will have two boxes... for instance a preamp. Direct lines, power conditioners, high end power cord all contribute to better sound. 

I payed attention to specs on equipment for the first couple years and then gave up, because by and large you get misdirected by measuring a few variables as there are so many. Hence, I never started measuring power or distortion. 

Having TV’s on usually reduces enjoyment. Seriously though, you have to try this out for yourself. Dedicated lines are a significant upgrade if that is possible for your system. I was listening to my system last night, Christmas Eve, and the sound was exceptional. Assuming there would be lower noise on power lines coming into my home due to the holiday, it made me think I might need better filtering. The folks that use the PS Audio regenerators seem pretty happy with the results, and regen seems like the ultimate resolution. 

Some knowledgeable people have said, for best results unplug everything in your video system - unplugged not simply turned off...it works in my system, and is obviously a free tweak...I'm on an excellent power grid, and power conditioning has only had a negative sonic impact for me ...

I wish I could tell you that any of these meters gave solid data on noise and perception.  So much depends on the filtration in your gear as well. 

My basic guidelines are to create two zones of power conditioning.  One is the clean zone on which your analog devices with nice power supplies run and another is the dirty zone for wall warts, TVs and PC's. 

The biggest mistake I see users make is to put noisy and dirty devices on the same outputs of a power conditioner.  With few exceptions you just introduce noise after it's been removed.

I didn't expect a definitive answer but you never know until you ask, right?   

I did find that my garage lighting is horrendously noisy.  It's cheap, bright 100W total LED strip lighting with a whopping 1,300 mv noise at the source, and a good percentage of that showing up at my gear.   

So garage lights off.  

Reducing line noise from 70 mv to 50mv (tv off) is beneficial.  70 mv is where the strength bar starts flickering a second bar.  50 mv and below seems sonically ok for now.  

Everyone's power situation is unique, some cleaner than others.

Conditioners very in effectiveness, likely a high quality conditioner will make a significantly noticeable positive sonic difference.

 to be worthwhile?  

 @wlutke only "you" can decide if it is worthwhile enough to part with "your" money

My old DAC was plugged into a power strip that it shared with the wall wart for my Chromecast Audio.
 

Sometimes, I would have people over that wanted to listen to my system, and I would unplug the wall wart mid-song. Every time, folks would sit up and ask "what did you just do?"

A while later, a lightening strike nuked my $8k JVC projector, and my NAD AVR.

I have since invested in 3 ZeroSurge suppressors that all have isolation. 1 connects to the HT sub and projector, another connects to the AVR and BD player and has built in USB out to replace all the wall warts. I later added the big 20amp 12 outlet to the 2channel system for the added mains filtering and to isolate the DAC from the amps. The one with the USB outs had a huge impact on the sound quality, the one connected to the DAC/amps had a modest impact provided the basement shop lights were off, and a big impact if there were on.

The moral(s) of the story are:

Wall warts and switch mode power supplies can and do have a very adverse affect on the sound quality of a good 2 channel system. Included in this is dimmer switches and (most especially after wall warts) is LED lighting. LED lights use the cheapest SMP's on the planet. I knew a guy who's garage door would open when he turned on the lights, and my GF had her thermostat make a buzzing noise from an LED lamp in the basement.

MOV based surge suppressors (this includes even the very expensive audiophile ones) fail silently over time from multiple "micro-surges" that occur everytime your fridge/AC/furnace/etc turns on or off. These need to be replaced frequently even if they seem to still be working. I knew the risks at the time, and it cost me $11k to replace everything.

Gear that pollutes mains should be isolated from the rest of the mains. ZeroSurge suppressors are expensive, but they never fail over time, and exceed filtering requirements (they are "lab" grade). 

YMMV