Joe, line length is a trimming tool, just like adding weight to the nose, tail, or wing tip, or moving the leadout position. Every plane/motor combination has an ideal line length. Too long and you will have problems with line tension, especially up high, and the plane will get whipped around by the weight of the lines swinging around in maneuvers. Too short limits the amount of maneuvering room, and of course you may end up turning around faster than you find comfortable.
Once you've got the plane trimmed so it is straight and flys OK on stock lines(60 ft for 35-40's, 65 for bigger motors, 50ish for 25's) make up a set of lines about 5 ft longer and see how the plane flies and re-trim it. Cut off a foot at a time and re-trim until you stop seeing any improvement. With some planes, especially the smaller lighter ones, as little as six inches can make a noticeable difference in flying.