"I might need it someday!" I hear this excuse a lot when confronting clutter. It's a real feeling, and I have the same reaction to some of my own things. That's why I have a container of containers- a place for me to keep those little bags and boxes I think I'll use for drawer dividers or something. Most of the time I use the containers I save, and they don't pile up and creep out of their master container.
One of my organizing heroes, Hazel Thornton of Organized For Life, has a fancy pants GRAPH that illustrates what to keep, what to toss, and how much space is recovered. She has examples of common situations, and she even suggests a bag of bags and box of boxes, which I already use! She explains everything I'd want to say in a beautifully simple way, so just go over there and check it out.