Could the different opinions regarding atemi be based on whether you train in an older style, like Yoshinkan, or a more modern style like Aikikai or even Kokikai? Perhaps even your sensi's philosophy?
It has to be because we don't use atemi much in Yoshinkai aikido. The only one we use usualy are small punches using the back of our hands. These punches are mostly directed to to face of the uke to gain the half a second.
I really would like to know though how the other styles use their atemi.