For fly fishing with rods five weight and lower, I am not that particular. I tend to buy whatever I can find on sales for the lower weight rods. Some of those rods cast better with standard weight rods, and some cast better with the "half weight" heavy lines.

My experience has been that the higher end Scientific Angler fly lines last longer than RIO or Cortland lines, especially if I use the lines for warm water fishing. Fishing in ponds or rivers with silt and sand dirties up the fly lines and Scientific Anglers fly lines are easier to keep clean and slick, so they fish well longer than other lines. That being said, I usually buy fly lines when I can get them on sale.

For heavier line weights where I am fishing with heavy, bulky flies, the fly line taper and design can make a big difference.