I use an APC H-10 unit that helps regulate voltage, I get minor swings/variances in my main system room and it certainly helps.
In our basement, which we 1/2 finished when we moved in, we have a newer 20-amp circuit, and I've had systems down there that need no conditioner and sound great plugged right into the wall.
I've also used Shunyata (currently) and Audience (past) AC conditioners that subjectively seem to make a subtle but nice difference; but I got them used on great deals, would never buy them at new prices.