If you only have ONE sidebar, then it wouldn't matter if it's left or right. If you have TWO, then the main bar would be the best one, and 9 out of 10, that would be the left one.
ohadgliksman is correct blending the ads with content is the best thing but placing ads in nav bar may give good results. you hav to try it out. in this forum too ad is placed in the header above the nav bar!