Many games feature Luck, gift dice or effort systems. I absolutely love Luck systems. The vast majority require you to spend these resources before the roll. The issue I have with this is one of risk vs. reward. You risk a resource that may or may not replenish, and you pin your hopes on attaining this one roll/accomplishment. However, you have no recourse if the one roll that you identified as being important to you fails. By and large, I allow players to spend these resources after the roll. It allows players to identify which rolls matter to them and allows them to achieve those successes on occasion. When I design these sorts of systems in my games, I make sure that players can use them after the roll as well.