I believe in atemi every time. However there are at least three scenarios I use and teach.
First and most basic is to destroy, injure or disoriented.
The second is to enter the body through the mind, Uke being cognizant of his environment recognizes the opening and moves to cover it and Nage can now control body and mind unified.
Third enter the mind through the body. Uke attacks with out regard and nage strikes a part of uke's body to draw the mind there and then applies technique thereby unifying body and mind and controlling at that point.
1. Destroy, injure or disorient
2. Enter the mind through the body
3. Enter the body through the mind.