In general, Grailing a Servant to 100 should only ever be done of you really like the Servant.
In terms of gameplay, it does help, but is definitely not necessary. Her HP increase does help make her even more tanky, and since she’s got Ruler Class Resistance, she basically receives double the amount of health. Also, Rulers tend to make fairly decent use of any increase in ATK because of their 1.1x damage modifier. However, the increase in her HP and ATK values between 90 and 100 isn’t high enough to ultimately make that big of a difference.
All in all, if you really like Jeanne (one of your favorite Servants) and use her a good bit, then by all means, go for it. She’s a solid Servant.
Edit: It should also be said that I’ve Grailed my Jeanne to 100 and I do not regret it.