Grail on Jack?

If you like her and will be using for a long time, go for it. Gameplay wise, Jack is already strong without grails. And Assassins aren't really advisable for grails due to their attack modifier which 0.9x compared to Rulers, Avengers and Berserkers which is 1.1x.

Jack's NP boost vs females and 50% quick buff means she can get more from the extra attack stats than most though being a 5* means you get a lot less efficiency per grail. So all in all if you like her go ahead you could do worse, if you goal is purely to use grails for gameplay gains you'll probably be better off looking to lower star servents who gain more levels per grail.