I personally don't think it's absolutely necessary to e-mail principals. Maybe if you get the initial interview and it's with a principal or whomever, then I can understand it. But if you don't even get an interview, I don't think it's worth it. The only time I went to a principal was in-person when I didn't have a FT job at that time. I had connections and a good sense of the area, and I was particularly interested in the school. But if you don't have a lot to stand on, many people would rather wait until you get picked out of a pile from my experience.
I've even worked with a person where if you sent that person stuff, that person was much more likely to reject you whether you were staff, parent, or interviewee.