Grailing servants :)

honestly the diff 99 times out of 100 between a level 90 servant and a level 100 servant is not going to matter and is only really going to be seen in overkill damage.
Unless you love a servant and want the level 100 bragging rights, I'd avoid wasting the grails on level 90 as again, vast majority of the time skills, CE's and team competition is going to be much more important than the 1000ish stat points an SSR servant will gain between level 90-100.

There are some 4 stars that when grailed to 90 have similar stats to 5 stars and for gameplay reasons those are popular for grailing, 3 stars don't scale to 4 stars as nicely but if theres a servant you like as a character but don't use often I'd say thats the one to grail as they'll appreciate the stat boost and you'll appreciate being able to use them more often.