Best practice is to ground your antenna system regardless of whether it mounts in the attic or outside.
The grounding of the existing cable service connection should not be shared, you want both the signals and grounding to be separate from your antenna system. Fault current may never happen, but if it does, you don't want that current to flow from the cable company into your antenna system or vise-verse.
Take a look at these resources:
http://ecmweb.com/code-basics/articl...sion-equipment
http://forum.tvfool.com/showthread.php?t=901