Common but good question.
For the most part, the smokes can go on any ckt....but, they can not have a switch to turn them off except for the breaker.
With that said....one good idea I got from this forum was to put them on one of the light ckts.....that way, your less likely to turn them off if they start to 'bother' you.
So...in my case...since I'm adding a bunch in my new addition...I put them on the laundry room lights...