Grailing 5 Stars vs 1-4 Stars.

I grailed Jalter to 100, and it is worth it for me. Even that many of my in-game friends has grailed Jalter, it doesn't hurt to max your own servants and it's not like I will be using double Jalter. Even the difference is only 1254 ATK from a Lv. 90 to Lv. 100 Jalter, this will help boost her damage. In the future if I rolled for Hijikata, I will also grail him to 100 for his NP damage boost depends on how low his health, like the gimmick of Anne and Mary. Avengers and Berserkers has 1.1x attack modifier, so they hit hard than other classes with the consideration of neutral damage. I tend to meta first over waifu, make the strong stronger.

Grailing 5 stars to level 100 gives, on average, around 1000 additional stats on both HP and attack. These stats are quite a significant amount especially if you factor in the various buffs that you will apply to your servants in later content (mostly damage dealers).

Generally people grail 4 stars to level 90 since they cost cheaper to grail (2 grails opposed to the 5 grails needed for 5 stars). The amount of stats gained from them are quite similar, averaging at 1000 for both stats. For some servants such as Heracles, these stats are beneficial as it helps them stay on par with 5 stars

1-3 stars are quite hard to pin down since they vary a lot in terms of stats. It's not really a good idea to grail 1-3 stars since they are very expensive to grail and the payoff is usually not worth it. An exception would be Cu FSN, which is arguably the best lancer in the game due to his durability, so he may appreciate the additional stats more than others.

I personally grailed Jalter to 100 since she desperately needs additional bulk for survivability, and extra damage always helps. You can really feel how the additional 1100 health really keeps her alive, ,although I can't say the same for her damage since she usually deals insane amounts of damage that it's hard to track the difference.

If you're looking for sheer efficiency, don't grail past level 90 - the stats gained per grail are just pathetic after that.

I've seen a lot of discussion on this, and most people just claim grailing is an act of love. If you want to use them for practicality, use them on common-use servants that benefit a lot from it, like Hans or Cu Chulainn, who don't have stellar stats because of their low rank, and will continue to gain 5 levels per grail for several grails.

I'm in the camp of "do what you want with them" since grailing from 90-100 is one of the most laborious, low-return things you can do in the game.

Grailing lower stars is worthier. 5 grails for only 1k+ ish stats is ridiculous, you'd better off using Fous. There is a reason why people recommended grailing your servants only to 90, it's for efficiency. As far as I concerned, people who went 'grail for stats' usually grailed their lower stars (3* and below) to add their survivability and make them comparable to 5* in the term of staying longer on the battlefield. Also attack stats really doesn't matter, lol. Your main way of doing high damage is buffs stacking and not sky high attack stats. It's the reason why supports like Waver-Merlin are highly regarded in the first place.

I grail for fun. Grailing for (nonexistent) meta is simply boring and I don't want to see copies of my grailed units.

For me, I would prefer to Grail non-5 star Servants. At least for now, the max-level 5 star Servants are more than enough to carry you on all of the future events, challenges and quests. For me, grailing is more like giving a room for survivability for Servants or those who benefit so much for having an extra HP.

Take an example of Hans, he might a 2 star but he's outright one of the best support in the game. Too bad, his survivability is pretty low. Grailing him will give him the needed extra HP for survivability. He will benefit so much from it. I know it's going to be a long grailing for him to get the optimum level. However, not everyone is so lucky to get Waver, Tamae and later Merlin. Having an optimum Hans in your field will never be bad.

Servants like Cu and Herc also benefit so much from grailing. They are, hands down, one of the best survivalists in the game. Grailing them will give them the needed means to compete against 4 or 5 stars. Seriously, sometimes even Cu could outlast Scathach under the right circumstances. Though, there also some 5 stars that might need Grail too, like Jalter as everyone else said because she really gets the benefit of extra HP. Okita might also benefit this because of her rather low survivability.

All in all, it's up to your choice on who you should be grailing. If you want to give it to your waifu or husbandos, then so be it. However, don't forget that we also still have Fou cards. Try to optimize the use of these cards before maxing your Servants. Basically, you will get a bonus 900-ish HP or ATK at Max for your Servants. This is will also give some extra stats other than grailing. Also, I don't recommend to Grail tanker like vanilla Jeanne because she's doing a pretty good job as she is right now. I don't see any reason to Grail her other than waifu aspect.