You can collect Sheriff Stars by breaking crates, killing enemies, and performing other actions. Spending Sheriff Stars does not effect this achievement in any way. I would recommend that you spend your Sheriff Stars on upgrades that boost Sheriff Star output e.g. more Sheriff Stars for killing enemies. More than likely you won't have unlocked this by the end of the game. You can boost your total Sheriff Stars by using the method found HERE. (Thanks to Suki). To use this method, pick "Tale Select" instead of choosing a new game.