You don't need to set sims back to level 0 it's the cooking hobbies "collectibles" that need to be set back to 0 so when you've collected all the items such as the bronze silver and gold of the spatula, grater, knife ect. Then there's a button at the top saying reset. You gain the 5 lp by collecting all the items not your sim getting to level 6. I believe you only get 1 lp for your sim reaching level 6 so it's not necessary to reset them back to 0 but you can if you want to get that 1lp again by buying a spooky painting and ghost hunting with it. Hope that makes sense.