Technically, hickory doesn't have true sapwood or heartwood. At least not something you can easily distinguish based on color. Instead, whitewoods such as maple, ash and hickory have what is called
false heartwood. Older trees develop a darker colored core, but the lighter colored wood is usually prevailing. The false heartwood may be slightly weaker than the whiter wood surrounding it, but it may not be necessarily too weak for a bow. Some false heartwood in your bow won't hurt and just adds character.
As long as you don't see any burrows left in the back of the bow, you should be fine. No matter how many rings you've removed, as long as the back is still in one ring more or less, it will be fine. Hickory is outstanding in tensile strength, so a backing will not be needed unless the back is violated by the critters or by your unproficiency in following a growth ring.
