If your centrifugal clutch is working right, at idle engine speed, off the throttle, there should be no engagement between the engine and the bike's wheels. Therefore your bike's speed should have nothing to do with engine RPMs. Downhill, off the throttle your engine should not overspeed. However if you're twisting the throttle just to see how fast you can go then the engine is connected to the wheel and it can be over reved. Brakes are a good idea.
Tom