check out "bowstick" on youtube...that guy has made a ton of short bow-making videos, and among them is one where he tests the strength of a hickory heartwood bow. Yesterday I talked to him and the guy from rudderbows about this very question. They both say heart wood is just fine, and they make lots of bows, so go for it bud. And if you do have a strip (or can get or make a strip) of hickory sapwood, so much the better. Floor tiller your heartwood board stave, and then glue the sapwood strip on the back in perry reflex. You'll be glad you did.
perry reflex is when you glue the backing strip on with the bow BENT BACKWARDS in an even curve from tip to tip.