It can be, but that doesn't mean you have to.  For me it depends on why I don't feel like running.  If I don't feel like it because of pain or soreness, I make a decision to run or not to run based off of how much pain I am experiencing.  So if it is pain related, I listen to my body, then decide whether or not I should do what my body wants.

If it isn't pain related, then I ignore my body and run anyway.  My body always ends up getting used to it and quits complaining, so I can finish the run.  If it ever gets worse, or screams louder, then I would probably call it quits for the run.

So yeah, it can be ok to run.  You just need to know what your body is saying and why it is saying it.  There is absolutely nothing wrong with taking a day off and running another day though.  If you don't know your body that well yet, playing it safe and taking the day off is not a bad idea.