You're talking about the bungees that stop the rear line from wrapping the wingtip?
Could be different because mine is a 2011 11m Rebel, but I haven't noticed any difference with or without the bungees. One of my bungees broke on one side. Flew with it broken for a few sessions, didn't notice any difference or pulling to one side.
Then I was lazy, never got around to replacing it, and just tied a square knot to put it back together. Effectively shortening it by several inches, still no noticeable effect.
Others have theorized in the past that these bungees help give the Rebel the short bar throw, or stop your lines from going slack, but I don't think they really anything to the flying characteristics because they are so loose. Who knows, if it works for you go for it!