What should I do with the Grails?

Hmm wel it actually depends of how much you like a servant,if it is your waifu/husbando I don't see any problem grailing them to 100.Grail who you love how much you like

For purely gameplay reasons, levelling servants from 70-80 and 80-90 will yield much a higher return on investment since it requires less grails than 90-100, and as for levelling from 90-100, the gain in stats is often fairly insubstantial, which is why people usually only bother grailing 5 star servants for personal reasons rather than gameplay value.

Grailing from 90 to 100 takes 5 grails...only do that if among your favorites there are Berserkers, as there is no such a thing as a Berserker with so many HP. Plus, extra dmg is allways good on them.

Oh well, Rulers and Avengers do have 1.1X dmg multiplier (and rulers resist almost everything, Avengers cause lot of dmg), so maybe they are also a good idea for the grail. Bigger damage rewards, plus tankiness.

For other classes, grailing 80-90 the elite 4*s (Chloe, KintokiRider, Saberlot, LancerAlter...) is usually the best, gameplay-wise; you make a quality 4* into a 5*, all for the price of 2 grails.

Assassins and Caster have dmg multiplier of 0.9, grailing them will usually give lower extra dmg return. So, just do that if you REALLY love one of them.