I think you are allowed and even recommended to use Arrow heads on both abscissa and ordinate. To me it increases the ability to understand the picture. More on this topic is here:

http://en.wikipedia.org/wiki/Mohr's_circle

http://www.google.com/imgres?imgurl=http://www.engapplets.vt.edu/Mohr/java/nsfapplets/MohrCircles2-3D/Theory/mohr2d.gif&imgrefurl=http://www.engapplets.vt.edu/Mohr/java/nsfapplets/MohrCircles2-3D/Theory/theory.htm&h=444&w=509&sz=6&tbnid=0GtEZxNKDxuJXM:&tbnh=78&tbnw=89&zoom=1&usg=__dNKw6W85B9222MGj2aZ6T3hP-G4=&docid=oD5DCmaXt0oz6M&sa=X&ei=b9gwUuuhHqa0ygHb04GIDA&sqi=2&ved=0CEAQ9QEwAw&dur=406

Hope it helps. :)

Thanks for the answer.

I think it's a matter of preference. The key is to make the drawing as clear as possible.