Is it a good idea to use Octave to teach an Intro to Matlab course?
As suggested also by SiXUlm in a comment, if you want to avoid being committed to a specific software, rename the course, e.g.,


  Introduction to scientific computing


or 


  Introduction to scientific computing with Octave/Matlab


During the course, outline possible syntax and library differences.


  when my students make it out into the real world they will most likely have to use Matlab


Most of your students will likely not use Matlab at all in the real world. A few of them will, but they would be able to fill the gaps by themselves.