Welcome to the forum. It depends on the pipe. It all comes down to this, if you change the amount of air flow through your motor (pipes and or air filter, head work and others) then you are changing your air/fuel mixture. New pipes are less restricting allowing more air flow. If you don't increase the amount of fuel your bike will run lean causing a bunch of problems(heat mostly). So get your new pipes and a screaming eagle air filter but then get a PCIII or a Harley race tuner and get her tuned up and your all set.
Some people say that if you just change your pipes or if you just change your air filter then you don't need a tuner. You be the judge. Ride it with the new pipes and see if she runs good or not. Check for popping on decel and excessive heat from the motor.
EZE is right. Harleys right out of the box are tuned lean to meet emission standards. You can purchase a power commander right from the jump, that way if you upgrade anything else, you can re-map. I have Vance/Hines slip ons with baffles and my bike runs extremely hot. You can buy the cables that will trick the ECU and change your o2 sensors from narrow band to wide band. Google nightrider and you'll find a lot of info. They cost about $136.00 a set and are plug and play. This will enrichen your fuel/air ratio...better throttle response, less heat, but your gas mileage will take a hit. I'm also running AMSOIL synthetic which also helps with the heat problem and has made a noticeable difference. HD pushes their products hard, but all the research points to AMSOIL being one of the best oil out there....Good luck!