If the selvages shrank - I clip them to the "regular fabric part" so the edges can spread. Then the piece will press "flat"
It is much easier to work with shorter lengths of fabric.
HOWEVER - check the directions for border lengths before cutting the long length into more manageable pieces.
I also prefer to have "stash fabric" in longer lengths.
If the directions call for a 60 inch border - I would allow at least 62 inches for wiggle room. I'd rather cut off a couple of inches of border than be 1/4 inch short.
I prefer to leave the clipped selvages on the big piece - but I do cut off the selvages on the individual strips or units.
That said, I usually have "ample" fabric on hand - but I can "make do" as well as the next one if I need to.