Depends on what you're working on, really. I find myself using stash when I quickly want to undo a change to check something, but preserve it for future use.

Stash is invaluable for when I need to do a hotfix or something or other and the work I'm currently doing on a feature branch is a mess of half-completed files that I just would not feel comfortable calling an actual commit. Stash allows you to save this and then come right back to it. Not every workflow needs to be followed to a T, at all times.

For example, to test partial commits:

    git add -p
    git stash
    test
    commit
    git stash pop