z track for all IM's?

My nursing school taught us to use the z-track technique for all IM injections although I've read from many people who were taught to use it only for staining injections like iron. (We were also taught to always change the needle on IMs and I've NEVER seen that done in practice). I did a flu shot clinic today and noticed the older nurses pinching for their IM injections, and have heard from some instructors that this makes it less painful. Just wondering what everyone thinks or if others are being taught to do blanket z-track for IMs. I used z-track all day and received many compliments on my technique however noticed quite a few also grimacing a little. Wondering what the rationale is behind this (we were given vague references regarding irritation) and if there is any evidence one way or another. Thanks for your input!