it is called self starting. Sometimes jobs that don't pay can benefit your resume, for example if MTV or Fuse or a network is casting you might want to go even if it doesn't pay a lot. And most magazine's editorial rates are not high, but the tearsheet leads to more work.
A model or actress with something on her resume will get more work quicker than a girl without one.
Be picky, but open minded.
Make sure you think of each job as an opportunity to lead to more and if the casting, modeling job or role isn't something that could lead to something else why do it? Still if the job isn't paying you hudnreds or thousands it doesn't mean it is a bad job. To build a reel you might have to take some chances and work without a high rate but hopefully it will be worth it!