I spent a winter in North Dakota with my first diesel. It saw -30*f. I used Power Service White Bottle or Stanadyne religiously.
That said, I will be headed up north in my current diesel in the next couple weeks to get an Xmas tree and I am not super concerned. I will probably put some additive in lightly, but 20*f isn't that cold if you have fresh fuel that is winter blended. In the cold months they blend diesel for colder weather. It doesn't matter if it is Phoenix or Flagstaff, they all get the same fuel to my knowledge.