Not to get anything started, but....I agree. I don't think ONE nail would hurt, just as ONE person's toliet paper above ground doesn't hurt. But if you give permission for one person (namely yourself) to do it, then you give permission for all to do it.
Personally, I do not want to see everyones used toliet paper above the ground's surface so I do what is "proper" with mine. In the same respect, I do not want to see a tree with a bunch of nails so I would not put my one nail in it.
A zip tie, string, wire, etc would be better and if you are a good geocacher, you will be checking on your cache periodically. When you check on it, you can see if the limb has grown any and the zip tie, string, wire, etc needs replaced with a larger version to reach around the growing limb.
Don't Think; Have Fun!