I wouldn't do it unless there is some sort of emergency, like running out of feeders and not being able to catch anything. I have let a couple of Carolina mantids have a taste of cooked, unseasoned chicken but they only took a bite or two before discarding it. :mellow:
I don't think that it would kill them or anything, but raw meat can grow bacteria that can possibly make you or your family sick, if you handle your mantid. I just don't think it is sanitary.
Insects are the best food for them, but I have fed a few mantids honey or banana as a treat. It is not natural, but amusing and fun every once in a while.