Does shooting a creature's head do more damage than shooting its limbs?

In Dead Space, you're encouraged to blow off creatures' limbs instead of just aiming for the body. Usually taking off 2 or 3 limbs will kill most creatures, but most games reward head shots more than limb shots. I'm not sure if that is the case with Dead Space, because of the importance of dismemberment in the game.

Are any of the creatures more vulnerable to head-shots than limb-shots?

The following excerpt from "Experience Points" gives some insight concerning your question:

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This theme of subtle novelty crystallized in my mind after hearing Jeff Gerstman's theory that "head-shots are ruining games." I think the theory has some merit: Many shooting-based games reward and incentivize accuracy through one-hit kill headshots to the detriment of combat variety. In many games there is little tactical advantage to targeting any other point besides an enemy's head and using any other weapon besides one that can deliver a precision shot. Jeff cited the latest Splinter Cell game as an example of how headshots work as a disincentive to experiencing the range of weapons and combat dynamics the game has to offer.

Dead Space addresses this problem by simply reversing the trend. The enemy "necromorphs" are most vulnerable to limb shots which makes fighting them vastly different to fighting most other video game enemies. The game's "strategic dismemberment" system rewards the player for experimenting with a variety of strategies and tactics: deciding whether to remove an enemy's legs to slow it down or whether to shoot off its arms to limit its offensive power quickly displaces the instinct to shoot for the head. The player must learn to study the enemy appendages, movement, and weapon vulnerability than simply finding the standard instant-kill spot.