Don't forget fair use. This wikipedia entry has a reasonable summary: http://en.wikipedia.org/wiki/Fair_use
That said it's harder (but not impossible) to successfully claim fair use when you use 100% of something, e.g. an entire comic or poem versus a one line quote from a longer book or article.
Also within a corporate setting it is much more important to pre-clear especially because profit motive is one of the prongs of the fair use test.
PS - to be clear requesting permission/pre-clearance is the guaranteed clean path to use, but it is not the only path.