No problem from me, but then again I prefer my own homemade helmet mirrors..... As someone else posted, whatever works for you, don't let other people dictate.
FWIW I have reflectors on my mtbs. Why? Because I go night riding now and again, and I'd rather be seen when I cycle up to the trails on a public road (plus a rear light as well). The number of times I've read in some juvenile cycling literature that it's seriously uncool just makes me laugh. But I'm older and I really don't care. The lemming herd can take care of itself.