J.G.
I will admit I have never baby proofed anything. I figure if your toddler is bending down the middle two tines of a fork and sticking it in outlets you have bigger problems than exposed outlets. I digress...
I have raised four kids in this house and only one of them hit their head on the fireplace. She was six, fighting with her older brother(she, as always, started it) and was pushed/fell against the fireplace. Two things came of this, one, a little blood but no scar, and two, the knowledge that it isn't a good idea to fight with your brother around the fireplace.
Especially at the age your kids are there is no reason other than misbehaving that would cause them to hit their head on a fireplace.
I guess another way to look at it is to ask how many times has anyone in your house spontaneously fallen to the ground and what are the chances of that happening right next to the fireplace.