I pulled wire for several years and primarily worked residential. I only ran 12 in the kitchen, bath, dining and outside outlets along with the garage and certain utilities. All lighting circuits were 14. Each bedroom was in 14 but each had a separate circuit. Hallways were tied in main bath lights and fart fans. Smokes were also picked up with general lighting circuit. Why run 12 everywhere when there is a significant price difference between 12 and 14. If you figure in a conservative load calculation you will be fine. Always remember to rate your circuit to 80% of full load. Example: 15 amp circuit should be designed to run 12 amps continuously. 20 amps kitchen,bath,dining,or garage figure 16 amps continuous amp draw. If I keep going. I might as well layout a typical 2000 square foot ranch with 3 car garage and unfinished basement.... just Kidding. This is a very basic description and always refer to your state and local codes. NEC etc...