There's no harm in riding your mountain bike on the road. I do it frequently and I own a road bike! It's a harder ride, that's for sure. Dh and I actually have an organized ride from our store on Sunday mornings that is a mountain bikes on the road ride. That's the benefit of having a mountain bike as your only bike instead of a road bike, it can go everywhere!

If you plan on doing it a lot I would recommend slicks to make it a little easier. Changing tires can get annoying though, so if you plan on doing a good mix of on and off road, I would just your tires.