Clearly there are some highly educated people in this thread, but I'll just throw my two cents in.... For my moms system we used Cat6 cable, all runs came in under 100 feet, I used Cat6 because I didn't really know much at that time.
For my system, I used the supplied cables, one wasn't long enough, so I went to HD and bought one of those ethernet cable connectors as a temporary fix until I put a new cable in. Haven't had any problems at all with my cables, my moms cables, or the little cable connector, so I'll probably just leave it in place until (if) something happens.
Now in the future when I actually build my house, I will likely use the best (affordable) cable I can find, and pre-wire everything up, so that'll likely be Cat6, just to future-proof a little bit.