It depends on what is defined as the "treasure" in your game and how much of that treasure the player deserves for completing the task. If your game is like Meat Boy, completing a level with a good time is the treasure, but there are also things to discover in some levels like Warp Zones and Band-aids, which are extra treats for the observant and skilled players.
If a game's focus in on battling and being quick at the buttons when up against a boss, then including a boss at the end of a level will be a good addition, because it will encourage players to improve their skills and reward them for doing well.
If a game is instead about rescuing a princess, obviously achieving that is the biggest treasure in the game, so it will only occur at the very end. But in the meantime, if players, for example, found letters written by the princess in the game each time they completed a series of levels, that would also be an achievable treasure that would work to reward players for getting closer to the goal.
I think any kind of treasure earned should reflect the main goal of the game, and higher and lower amounts of it should be awarded respectively. This helps direct players in playing the way you intend for them to play, and it rewards them for doing a good job at it. When they feel like they're accomplishing things, they'll return for more.