Originally Posted by World Tour
It seems to me that without it, the wheel would be out of balance since it has a valve on one side and nothing to counteract that weight. I imagine it would be more noticeable as your speed goes up.
A valve weighs only several grams, it is not noticeable at any speed reachable by a bike. A reflector is a bit heavier and can produce a detectable vibration if you turn the pedals very fast holding the wheel off the ground. In reality, neither the valve or a reflector can produce anything you can feel on the road. If you do feel something bouncy on the road at high speed, chances are it's your tire that's unbalanced.