WARNING: Tube/Valve amplifiers use potentially LETHAL HIGH VOLTAGES.
Building, troubleshooting and testing of these amplifiers should only be
performed by someone who is thoroughly familiar with
the safety precautions around high voltages.
Twisted AC is better than poor "DC" in straight lines... without a giant cap or a MOSFET pass filter (or an SMPS in the first place), most times you're better off with well twisted wire and AC current.
Twists will reduce/prevent both pick-up and outgoing interference, for DC heater wires only pick-up is possible of course as DC doesn't generate changing magnetic fields. Noise pickup on the heater is unlikely to have any effect for an indirectly-heated device.
It certainly makes the wires a lot stiffer and therefore easier to route. If the wires were thick (nice margin wrt to ampacity required) and solid (as opposed to straned) and otherwise thoughtfully restrained, I bet untwisted would be ok.
As was and IS good practice when running heater wiring, particularly with preamp circuits, twisted pairs AND routing/dressing intelligently is the best result.
Down against the chassis, away from signal wiring.
It's the way that its been done for decades.