Is it worth it to add substitute teacher work in a CV? As a graduate student, i've had to substitute teach a number of times for a variety of classes.  I'm wondering if it is really worth while to add this information into my CV.  Obviously, it won't hold as much weight as teaching a full class.  But if it is worth mentioning on the CV, what category should I add it to?  Teaching?  Volunteer Service?  Something else?
I am going to disagree with Peter Jansson here.

If you mean you stepped in to cover a couple of classes here and there when the regular instructor was sick or out of town, I would say this by itself doesn't constitute a meaningful amount of teaching experience for professional purposes, and shouldn't be listed on a CV.  It may have been valuable to you, but I think it would look kind of silly on a CV.

If you were assigned as a teaching assistant for the course, you probably have a line where you describe your responsibilities in that role (grading exams, holding office hours, etc), and you could add "occasional lectures" to that line.  

If you filled in for an instructor for a longer period of time, then you could consider listing it ("taught 3 weeks of Calculus 4").