It all depends is the Defiant assigned to the station or to Sisko?
is generally assigned to the station. So the ship is commanded by whoever DS9's commander says - In this case, Sisko. If he feels like giving Kira the ship, she gets it, and all questions are moot.
IIRC, though, after Worf arrives, he usually gets the Defiant
. A notable exception would be "Trials and Tribble-ations" and that's because Kira - as a Bajoran - would have no reason to sneak aboard the NCC-1701 (Dax was a more obvious choice to do that because she is an expert on that time period and Kira isn't).