They certainly have an effect. Steel hoops tend to ring a bit when drums are struck, which adds some high frequencies into the sound. Wood hoops don't ring per se, but add to the drum's overall resonance -- they are almost like an extension to the drum's shell. I've had the opportunity to compare my kit to my friend's Kumu kit that features flanged steel hoops, and there's a clear difference. The steel hoops give more attack, projection and high-end clarity, while the wood hoops give more roundness, warmth and darkness -- in a word, more "wood". Both kits sound great, of course, so it all comes down to personal preference. Darker sounds appeal to me, so I went for the wood hoops.