In truth, a large amount of this is "lore" and tradition. Many British amps have the heater wires run simply in parallel, untwisted, though close to the chassis. When building amps we really just want to place all the odds on our side. Fenders, of course, have the traditional "up in the air" twisted pair; yet with no regard to the phasing of the wires. I say "no regard" because both wires are green...and if you think that in Fullerton, CA, Consuela holding 100 feet of doubled-over green pushback wire while Fernando held the other end in the drill chuck in Leo's shop...they took care to produce coherent phasing all down the heater line....I think you'd be wrong. There are four components to trying to reduce heater-induced hum. 1: Center tap, real or synthetic. 2: Close to chassis, 3: twisted pair, and 4: phasing. If you do all four, you've done pretty much all you can (short of creating a DC heater supply) to reduce hum. The center tap is for the most part, mandatory. The choice of "up in the air" is, IMHO, more related to whether you solder the heater wires BEFORE everything else or AFTER everything else. My suspicion is that Fender wired up the heaters AFTER everything else, but I have no way of knowing that for a fact. If BEFORE, and you use different colors, then you can do 2: close to chassis 3: twist 4: phase; all at almost no extra effort. Experience has shown that the more of these you got going, the better. And that's about all there is to it. You want to use sufficient gauge wire to a pile of power tubes. It's just a "best practice" thing. Do it right, or as right as you can, and then you can pretty much stop thinking about it.