During a fire building process, can heartwood be used as a fuel for the fire? Assuming you are working with less than perfect materials, there is always a danger that your fire will go out at the point where your tinder burns up. Hence, when using heartwood, do I need to keep an eye out and add finer shavings of wood to keep the fire up and burning?
Heartwood, assuming the definition on Wikipedia as basically just the middle of the tree that is no longer growing, is indeed what you will be burning most of the time as fuel for your fire.
Considering it has not been growing for some time, it may well be somewhat drier than the surrounding sapwood, and therefore actually burn better.
That said, the overall size of your fire-making materials, and their relative dry/damp-ness is probably the most critical factor when making a fire.