I don't know how critical it is. The only times I've exceeded a rod's line rating was with 6 lb braid on a baitcaster with a 4 lb rating (only because the 4 lb braid was horrendously expensive). I don't know that any of the rods broken by customers were broken because of stronger than recommended line. Car doors, tree branches, getting stepped on, etc seem to be bigger risk factors. Still, I would suggest following the manufacturer's recommendations. If you need heavier line to keep fish out of timber you probably need a heavier rod that can handle that line anyway.