Actually, the talk about MRV (Multi-Room-Viewing) is that you'll be able to do it over the existing coax. Or at least, that's what was being talked about a couple of years ago. Not sure if that's still true.
That said, you can never have too much copper in your house!
At a minimum, I'd install 4 runs of RG6 (coax) and 4 runs of cat5 to each location you might expect to have a television at. You don't have to use it all right away, but it's nice to have, just in case.
I'd then run 2 RG6 and 2 cat5 (minimum) to other locations in the house - office, behind the sofa, etc. Cable's cheap, and when the walls are down it's easy to run them - so why not? No matter how much you run, you'll always wish you had run more....
I'd also highly recommend installing conduit to a few key locations, so that you can string new cables in the future in case some get damaged, a fancy new technology is released, etc. Even PVC piping is fine. Just make sure you put a string in there, so you can tie it to the new cable and pull it through in the future. It's a lifesaver. I wish I had done that in my theater, but I thought, "Eh, I have enough cables here, I won't need more." Then my HDMI cable broke.