I strongly recommend against grailing low-stars for power. Yes, they will improve, but all of them are easily outclassed by gold servants, which you will get eventually. The exception being Arash, but I have already explained why I don’t think he needs grails.
I also disagree with the philosophy of using grails just for the sake of using them. FGO is not a game you can beat, and thus any limited resource needs to be invested in such a way as to bring long-term value over an indefinite timeframe. If you grail a low-star to help out your early game, those grails are gone forever. It only takes one or two lucky gacha pulls to render units like Spartacus obsolete, and who knows how many servants you’ll discover in the future that you’d love to give your grails to.
Only grail servants you really love and want to use as much as possible. As your roster grows, the value of any given servant diminishes, and grails for power purposes become less and less impactful. Save your grails until you find a servant you believe deserves them, without reservation.