Does Taking in the Waist of Trousers Require Taking in the Seat?

I just dropped off a suit at my tailor and this is the first time I am having him taking in the waist of trousers. He told me that to take in the waist, I should also slightly take in the seat so that it avoids bunching. Is this correct?

I usually have a family member take in the waist of my trousers because she won't charge me. But she never took in the seat.

Does Taking in the Waist of Trousers Require Taking in the Seat?