I'm pretty sure I read somewhere (probably either Mors Kochanski's 'Bushcraft' or maybe Cody Lundin's '98.6 degrees') that fire rings are unnecessary, destructive to the environment, and actually increase the risk of a forest fire. A ring of stones may trap the heat in between two stones and cause a fire to emerge days later; several forest fires have supposedly been traced to this. (Of course, if you're conscientious and scatter the rocks after you're finished, this is probably less of a concern.)