If you are only going to use an image for your own personal use, then asking permission is not usually an issue. However, if you are going to post the image on a website, a public tree, a blog, etc. then it’s a good idea to ask permission.
One reason is that it is the right thing to do.
Another reason is that the photographer may have additional items, better photographs, etc. Irritating them may make them less willing to share information with you.
[note; The following paragraph was somehow deleted from this post when it originally went out.]
The best reason for not using a picture that you did not take is that the original photographer has copyright to the photograph that they took. It is their picture to use publicly as they see fit, not you. Whether the item they photographed is easily in public view does not matter. Whether it is the tombstone your great-grandfather paid for does not matter. What matters is who took the picture.
I have usually had requests to use FindAGrave images granted. But remember it’s up to the photographer.