From my personal eperiences, it all depends on what weapon you are using. I will just tell you by the weapons I use.
I use the shotgun. For the shotgun you dont need to aim at the head. Just te body to get a 1 hit kill.
The smg shoot fast and is easy to get head shot especially up close.
Use boost for match types like Assassination, Search and Retreive, and capture and hold. Use boost for tactians aswell.