Does a 450cc motorcycle need back-pressure? And if so, how do I know how much to give it?

It won't hurt a thing whatever type of exhaust you end up using. However, different pipes/mufflers may require carb jetting to be changed in order to gain the best performance. A pipe that freely flows may cause an excessively lean condition reducing performance and be potentially harmful to the engine. Be aware that more noise does not equal more performance.