The only time where arimidex must be used during PCT is when estrogen has not been managed properly during the cycle. In that particular scenario, 1 mg Arimidex ED for 10 days consecutively from the start of PCT will bring estrogen levels back into physiological ranges.
Arimidex and Nolvadex both serve different purposes. The distinction is primarily due to the fact that you want your estrogen levels to return to normal as well. Considering Arimidex inhibits estrogen production, it would not be the most optimal choice. Estrogen has many benefits such as increasing HDL and IGF-1. For example, the benefits of testosterone on GH/IGF-1 secretion will not be present once the supraphysiolocal levels have diminished. As estrogen levels increase, GH/IGF-1 does as well. It's not a huge issue since it's not highly noticable, but goal of PCT is to restore your body to homeostasis.
Jenetic