I had a hard time collecting information on smoke alarm placement. The best guidance I found was from the National Fire and Protection Association, but it does not get very detailed about placement. Here's what I found as electricians started drilling holes for wired smokes as required by local code.
What I did:
In the bedrooms I put them 12" off the wall, centered over the door, where framing allowed. The requirement/best practice seems to be 4" off a wall, if placing on a ceiling. They could be in the center of the room but all rooms have fans. I think code would allow them to be near/under fans but NFPA suggests it should not be done that way.
In the shared kitchen/dining/living room (vaulted ceiling), I put one near the apex on the living side. There is some guidance on placement for slopped ceilings and 10 feet from kitchen stove, but it is not more than 3 feet from the tip of the fan and I didn't read that until it was installed. Of course the fan will interfere affect the path of the smoke to the detector. Ideally it would have been closer to the bottom of the stairs but that is 7 feet from the stove, so too close.
Don't count on the electricians to know all this. The code is way looser than what best practices would suggest. Oh, and they said code did not require a smoke detector in that room, which I think is wrong since NFPA says there should be one on each floor. Their argument was that is was a kitchen, but it is also a shared living/dining room and as long as you are 10 feet away from a stove it is fine.
Another I didn't get quite right was the upstairs hall where perhaps it should have been closer to the top of the stairs but 3 feet off the bathroom door to prevent false alarms from steam (according to one source, not NFPA).
Since these are wired it's expensive to change the location so do your research and hopefully you'll have more time than me.
Other sources:
Nest is showing about 8" - 12” from wall. But NFPA doesn't say anything about distance from wall, or I missed it
3' from the forced air heating and / or cooling system air supply vents
Article Source: http://EzineArticles.com/3949593
Smoke detectors shall not be installed within a 36 in. (910 mm) horizontal path from the tip of the blade of a ceiling
Place alarms at least 3 feet (0.9 meter) horizontally from bathroom doors.
In basement on the ceiling at the bottom of the stairs leading to the next level.
Avoid Near fresh air vents, ceiling fans, or in very drafty areas. Drafts can blow smoke away from the unit, preventing it from reaching sensing chamber.
