Your temp consideration seems to focus solely on the water temp. Needn't one consider ambient temp change as well? In normal case prob nothing to worry about. But what about power & furnace loss?
If you aren't running water, the pipe and water contents will eventually reach the attic temp. Hopefully, you've got it in conditioned space (i.e., close to the ceiling under the insulation), but depending, it could rise and fall a lot from when water is running through it. You need to take into account the total maximum difference to know what amount of expansion/contraction you need to account for.
Your temp consideration seems to focus solely on the water temp. Needn't one consider ambient temp change as well? In normal case prob nothing to worry about. But what about power & furnace loss?
My temp consideration is based on abient temp changes. Note my avatar, I live in Hawaii. In Hawaii, it never gets down to 50 degrees in our area but I used that temp as a worst case cold temp. The 120 degrees hot temp is what our water heater is set at.
Everyone who installs pex should determine the worst case temperature extreme for the particular area that they live in.