If you angle your shots just right, occasionally you can get through levels without using up all your shots. You're awarded a bonus if you can do this.
Bounces
If you fluff up the first half of a level, just restart. You will not make it through if you have used up too many bounces in earlier sections.
Scores and times
If you really want the highest scores and the fastest times, go ahead and buy that bounce upgrade pack. It is the only way of achieving everything without absolutely tearing your hair out. That said, you certainly do not NEED them.
