I do have to say that burning/decaped zeds often are a threat. On most official maps you dont have much space to move one and then one grab by a clot, crawler attack can be devastating. And it's only crawlers that really burn out fast. It has happened a couple of times that I kill of the last gorefast with a fast knife cut. And straight after it's been decaped I often write something in the chat, and a couple of times that walking body have killed me in 2 attacks.
I can say that I've got only bad experience of not shooting burning/decaped bodies^^
So on suicidal I often think that extra bullet is worth it, but on everything below it doesn't really matter.