For my first couple _years_ with Git, I just passed the -a flag in upon every commit. This is a pretty easy, low effort habit to form, so this reduces to a question of which default to set. Git's approach seems to be that the default should be to nudge users toward power usage, which I don't think is necessarily a bad thing. (Note that git's approach to defaults in other areas, like potentially destructive actions, is actually a problem).