Is it worth grailing 5 Star Servants?

grail for love, grail the servant that you won't regret to have it and uses him/her frequently because grailing them is very costly and the stat growth is just around 1k+

Grailing doesn't really have a huge impact on gameplay aside from a little stats boost (which in my opinion, you can also get from maxing fous on a lv.90). SSR Zerkers in general are very good grailing targets due to their class.
Grailing SSRs are expensive, so I suggest you grail those you love and use a lot in farming and quests (mainly grail the ones you love).

The most valuable aspect to grailing a 5-star is that they appear higher on people's friends list. The QP expenses are incredible with very little in gameplay return.

That's why people usually say to grail for love. It's always nice when your favorite servants can pack a little more punch.

As someone who have grailed cu alter to 100 i would say he is probabily the single best servant to do so.
Still, despite thet, is not necessary or even thet usefull.
in general, grail is quite pointless for most servant.
by the way, nice line up

I do believe that only Rulers, Avengers and Berserkers 5*s are maybe worthy of grail (1.1X multiplier dmg, zercs are fragile, also), gameplay wise.

Otherwise, I would recomend grail 4*s, specially Herc, Saberlot and wellfares on NP5...