Let's say you have to run some non-rigid tubing in the wall - PEX, PolyBut,
whatever. The troubl is that it is running up, not horizontal and so you
need a way to stabilize the tubing, at the very least so that it doesn't sag
and/or fall back down while you're trying to lay it out. How would you
stabilize it permanently, assuming that you could NOT lay it against the
stud but it has to go in the middle of the space between the studs.

Would it be a good idea to simply run it so that it does lay against the
stud, or is it perfectly fine inbetween studs? If it is fine, what would
you use to keep it stable and somewhat rigid?

This is a problem that I'm faced with constantly, as none of my water spouts
come out anywhere near the stud, but because they are galvanize it didn't
really seem to matter - the pipe was rigid enough to hold it in place. With
the PEX that I'm replacing it with, it seems to me at least that the PEX is
too flexible and when securing a spout I'm not sure how to attach it to the
drywall and not have the PEX sort of hang out there inbetween the walls.
Something in me wants to secure the line to the wall so that it won't move
around or won't twist and rotate in the wrong place as PEX wants to do
sometimes.