You should only use as many as you need. If you need the FC for a purchase, claim them. If you want to sacrifice some claim as many as you need. It's best to at least partially level them up to maximize their essence value. You need to decide which sac event best suits your game strategy. You can always save for a later date.

Remember to spend your FC wisely, because you never know when you may need some to complete a spending event. The game has become one of resource management and not one of spend as soon as you can.