A dedicated line is usually substantially more voltage stable. You're not going to plug a vacuum into it because it's a dedicated line. That's the whole point.
And how often do you do that while listening? :D
Yes, it's more stable, that doesn't necessarily make it stable. I used to spend all my time listening and watching the voltage meter. Seasonal variations, plus major appliances in the home kicking on and off still make it more variable than using a voltage regulator.
Anyone else who is curious should get a Kill-A-Watt meter or some other constantly running voltage meter to see how well or poorly things in their home are. Be grateful if you find your home AC is stable all year long.