I just recently built a house and here is what I would recommend:
1) If you are comfortable doing it and the builder allows for it, do it yourself. Caveat: A good starting design is key! Trying to figure it out as you go is not a good idea.
2) Whether or not you do it or a contractor does it, take pictures and measurements before the drywall goes up. Our contractor made a mistake when cutting a hole in the ceiling for a speaker only to find he was off by 2 feet. I had to live in a "bubble" while they re-texturized my entire main floor. Knowing where the speaker wire is terminated behind the wall is important when trimming/cutting after drywall is up.
3) Consider conduit. I did run some conduit from my main basement stack to my entertainment room stack but I wished that I had run more conduit to more locations in the house.
An advantage of conduit is that you can pull some wire after drywalling is complete. Speaker wire can easily get cut/terminated due to a drywall nail hitting it - which is the case in my house. My left surround speaker wires are both terminated 3 feet up the wall from where they should be terminated - most likely due to a nail. Now I need to punch a hole in the drywall, splice/patch the speaker wire and re-patch the drywall...what a pain.
I would use 16g wire for audio distribution and 12g for home theater/surround speakers.
Good luck!
Bryan
