If you have been given a late clearance, a large intercept angle or you are flying a high approach speed aircraft with bank angle limits, there is a risk that you will exceed half scale deflection on the opposite side before you become full stabilized on the localizer. Calling established as soon as you first reach half scale deflection and then busting out the other side before settling down would definitely not be established. In my former life in military aviation not only did we have to satisfy the half-scale, but we also had to be within (as far as I can recall) 10 degrees of the approach axis before calling it. I still apply that principal to avoid embarassment.