>>530806912
First you should get your Grand Servant lineup to 100. That's one of each major class, and probably one of either Extra Class grouping (Avenger/Ruler/Mooncancer/Shielder-Paladin and Alter Ego/Pretender/Foreigner/Beast).
Then your meta servants to 100, sure. And then whoever you just like. I went around spending grails on people who I consider "family" in Chaldea for example.
Do what makes you happy, just bee yourself etc.