Excuse me if I am using the wrong terminology, but I just got into 3D printing and I am still attempting to calibrate my Taz 5. The print I am currently using for calibration purposes is this 15 mm cube (http://www.thingiverse.com/thing:25093). The vertical surfaces are not very smooth and from a few google searches, I think it could possibly be a wobble issue within the Z axis. I squared up the X axis with the frame, as best as I could, but I didn’t see a noticeable effect.
Here is a picture of my latest print, with the X axis squared.