I almost NEVER go any smaller than 15, and my success tells me that it makes NO difference.. hehe.. I'm sure that it can make a difference, but another thing that you can use, that I've found makes a little difference, is matching line color with the water conditions... I've found clear line to be good for almost all conditions though.Casting isn't much of a problem for me, as all of my reels can throw a #3 spinner, across any river I've ever fished, with 15# line... Hope this helps a little...