Never throw any sort of ribbons away!
If you look at some of the b'day cards and even crafts that I've made, many of the ribbons I used on them were what I've kept from scraps and gifts that I received. (or didn't receive - I take anyone's ribbons if they're about to throw them away).
There's always something you can do with "second-hand" ribbons!
I even keep those little ribbons that you find on your clothes, that you're supposed to use to hang them up on (only the ones I don't need to remain on the clothes)...you know...these things: