After repointing a fieldstone foundation, should I backfill with gravel?
That sounds reasonable. The goal is to keep water away from your house. If you've ever stood on wet soil, you know that it is soft and has little strength. Also water under your house can lead to mold and mildew.

The gravel makes it easier for any water to drain down so the area under your house stays dry. Without the gravel, water would also try to flow through your foundation wall and go under your house.

The graded topsoil encourages rain water to flow away from your house.

If you like, you can even put a plastic sheet in two places:  


between the gravel and the foundation wall to help block water going under your house.
between the gravel and the topsoil. Make sure it is also graded so water drains away from your house. This would really be your primary defense. The first sheet of plastic would be acting as a backup if you have this.


Did you have any specific concerns about this approach or are you just asking for general confirmation?