Should it be just that. Once you're laid to rest, that's it. How would you feel if a builder was suppling stone for work around your house, that when turned over you realised they were once used to mark a persons grave. Many churches now seem to use gravestones, that used to mark a grave that is still within the grounds, as cheap paving. I try to avoid where possible walking on the gravestone/headstones/memorials out of respect for the dead. Extending this to where they have been used for cheap paving. One nearby church has had some of it's walls, within the grounds, rebuilt entirley from gravesones, cut to size.