Which Servant worth grailing?

A lot of people probably Grail Jalter because she has a 1.1 attack multiplier, so she can take advantage of the extra attack stats that she gains better than some other servants (i.e. if you raise her attack by 1000, it's actually like you're raising it by 1100). Also, they probably figure that they'll use Jalter a lot, since you generally use her without regard to class advantage. It's obviously best to Grail servants that you intend to use frequently, rather than servants that just sit around and collect dust. These are the same two reasons that people often Grail berserkers and rulers. As far as why they Grail their SSRs, even though it's considered less efficient, there is a decent reason.

On one hand, it makes sense to Grail lower rarity servants, to bring them up to a level where they can somewhat compete with higher rarity servants. After all, it costs more Grails, more QP, and more exp cards to Grail an SSR.

But on the other hand, if you grail a lower rarity servant, you're running a higher risk of investing Grails in a servant that you might be replacing in the future. For example, suppose that Ceasar is the best saber that you have right now. You *could* grail him up to 90 to make him even more viable. But then what if you wind up pulling Saberlot or Altera the very next day? At that point, you just wasted 4 Grails and a ton of exp cards. That's why some people choose to Grail their SSRs; while it may technically be less efficient in terms of cost, there is way less of a chance of wasting Grails, because it's way less likely that you'll wind up replacing that servant any time soon. This is especially true with a servant like Jalter, who can be used against any class and will grow even more valuable in the future (once we get Merlin for phat buster crits).

If you like Medea and use her a lot, then it's not a bad idea to Grail her. But at the same time, if you only use her in the rare circumstances that you need a spammable buff removal, then I personally wouldn't grail her. Especially since she doesn't even need extra levels in order to do that job. A level 30 Medea can strip buffs just as well as a level 100 Medea can, as long as you can keep her alive, so why grail her? Plus casters have a 0.9 attack multiplier (if you give her +1000 attack, it's really like you're only giving her +900 attack), so they generally gain less from grailing than most other classes.

Btw, I'm not necessarily advocating that you grail your SSRs. I'm just explaining why some people do it. It makes more sense than a lot of people give it credit for.