In MY own home theater, I'd only use oxygen-free, double-twist, reverse-osmosis, beryllium-plated cable with solid gold end connectors with iron plating on those -- but only if it costs $300 or more per foot so that I know that it'll do the system good.
Geez. I've used telephone wire, 300-ohm twinlead, lamp cord, RG-58, RG-59 and RG-174 coaxial cable and heaven knows what else for speaker cable -- short distances and 50-foot runs. I'd challenge anyone to a double-blind test on speaker cable with medium-scale speakers if the cable is at least 24 gauge for a short (less than 20 feet) run and be able to tell the difference between types of cable. If you're running massive power to the speakers, then the gauge can become an issue just so power gets to the speaker and the damping factor isn't buggered up too badly. If it's a 200-foot run, the gauge can figure in. But a room? Nah! The CAT-5 cable should do nicely, even for the front speakers and the subwoofer.
My only concern would be if you have a house with all different kinds of cables running throughout: speakers all over the place, intercom systems, whole-house vacuum system, TV RF cables, thermostats, telephones, computer network, etc. Then it might be nice to save the CAT-5 for the network, telephone wire for the telephone, zip cord for the speakers, RG-6 for the RF, thermostat wire for the thermostats and vacuum system, etc. just so you can tell one system's wiring from the other at a glance. For example, using 12-2 with ground Romex for all your various types of wiring would be confusing and dangerous.
Dean