Go ahead and share anything that's yours, meaning anything you have the legal right to share with others.
Usually this means documents or anything you made yourself, like home movies, photos, or music you wrote. Don't share anything for which you don't own the copyright, unless of course you have permission from the copyright owner.
Buying and downloading music and movies on the Internet does not give you the right to share them with others in a cubby. Naturally you can keep your music or video libraries in sync across your own computers. When in doubt, you should probably just avoid sharing downloaded music and movies via Cubby.
In short, you can keep your own stuff in sync via Cubby, but don't use it to distribute content you don't have the right to distribute.