i use 14 in every residential job, and i save mucho$$$$. i don't know where some of you get the idea that it will take many more home runs, labor, etc. . . but that has not been true in my experience. also, you can't put the smokes on just any 20A circuit. they have to be on an AFCI circuit.
in a typical <2k 3 bed home, i have the following;
15A - 2 guest beds - afci
15A - Master bed - afci
15A - bedroom lights, smokes, - afci
15A - lights
15A - lights
15A - garage & outside receps on gfci recep
15A - overhead door opener, doorbell, attic lights, etc..
15A - dishwasher
15A - microwave
15A - refrigerator
15A - living rm, hall receps, etc. . (usually about 15 +/- receps)
20A - kitchen receps
20A - kitchen receps
20A - washing machine
20A - guest bath
20A - master bath
20A - dining/nook receps
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that's it for the 120v stuff, i may have missed one or two, but i don't see any extra home runs that had to be pulled because of using 14ga. even w/ 12ga i would split the bedrooms like i do. the joints are easier to make, the devices can be backstabbed. its a no-brainer. the material cost difference on this job would be around $150-$200; the labor difference is more significant.