Definitely kill Necromorphs
Necromorphs love to jump the player either by playing dead or leaping out of vents. Not every vent will spit out a Necromorph, but enough will do so that you should always keep a close eye on them. Meanwhile, any Necromorph lying on the ground that you didn't kill is almost certainly faking it, especially if they haven't been cut to pieces yet so go and make sure they are dead.