I put them on frag racks if they're very small and allow them to grow a little (or a lot, sometimes they stay on there for months due to lack of time to mount them). For the most part they usually just encrust onto the frag plug which is a pain but then I whack the bottom off and epoxy it to the rock much (much) later. otherwise it usually gets lost in the tank as a small frag.
For the larger colonies that come on those chunks of rock I usually remove them and mount them directly to the rock. For some reason I can't throw the rock out so it ends up in the sump, spreading its lovely hitch hiker wonderfulness to the rest of the tank
