IMO, if you have the chance to run wires now before a new floor gets put in - do it! That's just my two cents. You never know what you'll want in the future, and if you can have the wires already routed, I think it's well worth it. I say think about what you might want to put back there eventually, including maybe a sub (so perhaps a 75-ohm cable).
BTW, I take it that you don't have a basement running underneath that room. If you did, you could probably run wires fairly easily.
When I had my new carpet put in, I ran eight conductors of flat 16-gauge speaker wire. I only had two speakers back there, but I did it in case I decided to either move to a 7.1 system or bi-amp new rear speakers (that I never got). I did add a rear center channel, so I'm happy that I had those extra wires already run. I bought Gecko wire (and wrote a thread on it
here). Their site is currently under construction, but you can buy similar wire elsewhere.
EDIT: Hmmm. The Gecko cable site went from under construction to not in service as I was clicking on it tonight. The cable from this site
here looks to be the same stuff.