I use Dimension3D's tools for this with no glitches on all kinds of geometries :)
Also, I use UV mapper to get around having to mess with groups (not sure if Zbrush takes care of that or not)

Hi, my namez: "NO, Bad Kitteh, NO!"  Whaz yurs?
BadKittehCo Store  BadKittehCo Freebies and product support