Tara,
You raise a good question but one which the family members of those gravestones you photography must be aware of also. Since gravestones are mostly in "public cemeteries" i.e., those accessible to anyone wanting to enter, and the cemeteries do not set rules against taking photographs then you should feel free to take pictures of whatever you want. Limit only those in "private" cemeteries and getting permission to enter them you should also request permission to photograph all stones.
Getting "permission" may not be easy. Sometimes it's difficult to find the owner(s) so use "reasonable" efforts to find them by asking local land owners, libraries,genealogical organizations etc. as to the owners. All this should give you some legal coverage but personally, if there's no lock on the gate or no gate then then access is "open" and you should be able to take any and all you'd like.
I see it as I do my own searches, I wish someone back a couple hundred years ago could have photographed my ancestors graves ! I'd be very happy if they had ! If someone "Posted" my parents gravestones today I doubt I would feel upset about it. More than likely I would be proud they did ! How would you feel about your own relatives? that's the best tool to use taking your own photos!
Good
Luck, clear shooting, and sunny days to you.
Rick
Foulkesrickster45mi@hotmail.comP.S. I don't usually reply to "opion poles" but I take cemetery photos too! So, too add to my farewell...
Long life to your batteries !
Rick