Hi stormtoad and welcome to RCU. Please do not be offended but I must differ with that statement. If what you mean by "being tail heavy" is having the center of gravity to far aft then that definitely will affect level flight. The airplane will become unstable in pitch and what will happen every time is something the engineering types call a 'pilot induced phugoid oscillation'. This oscillation is directly proportional to how severely aft the CG is and it can range from just unpleasant to completely uncontrollable.

John