I agree that headstones are placed as monuments to stand in rememberance of people now gone. They are meant to be seen. I can see no disrespect or poor taste in Photographing them.
I can't say that I have taken a whole lot of photos in Cemetaries but it seems an awful broad theme to abandon out-of-hand as being derivative. With so many of them around the World, each with it's own character and full of unique detail, it would seem a fertile subject for original visual and emotional interpretation through Photography.