"Why aren't you spending your grails?"
"What if DW comes out with a servant I love even more?!?!"
FGO has given us ~19 grails thus far. We have enough to max grail 1 or 2 servants and still have enough left over to grail any new ones DW comes out with.
You don't have to grail anyone but I...
Always grail for love. No point in grailing someone you don't enjoy using. I grailed both Marthas to 90 and JAlter and Archuria to 100.
I find grailing to 100 is a huge investment. Not only the grails but also the EXP and QP you have to spend.