My knowledge of hellhounds is not extensive, but I do know of them in British folklore where they are considered to be omens of death or illness, they appear before the one that is said to die or become seriously ill. How literally you choose to take that is completely up to you. Because of this tradition of hellhounds in Britain, I personally wouldn't think they would just be found in graveyards or at crossroads, because the people at a graveyard are already deceased.
I also agree with Lark, what do you mean by a "witch graveyard"? In the majority of cases a witch is buried just as a normal person would be because most of the time a witch would keep their practice solely private, at least around here we don't have graveyards for specific practices like witchcraft, they are just buried with non-witches.
As I see it, witch graveyard isn't the graveyard with buried witch(-es), but graveyards which are used by the witches because of its high necrotic activity.
Hearing or even seeing dogs (so called "hellhounds") is usually a very special omen, it means that Governing Power of graveyard (Death Matron, Black Widow, etc., this Power has many names and faces) took attention at your presence and actions. If no respect was shown by the person, this omen is very bad. For experienced witches, who have established a good contact with the dead, it usually means a good omen, their rites/spells will succeed.