A rule of reason applies here. The impact of imbalance depends on the amount of imbalance and the rotating speed of the wheel. Given the working speeds of bicycle wheels, I doubt you'll ever notice a functional consequence of the amount of imbalance introduced by the reflectors, and wouldn't worry about it.
__________________
FB
Chain-L site
An ounce of diagnosis is worth a pound of cure.
Just because I'm tired of arguing, doesn't mean you're right.
“One accurate measurement is worth a thousand expert opinions” - Adm Grace Murray Hopper - USN
WARNING, I'm from New York. Thin skinned people should maintain safe distance.