Takeoff. The engines are operating at a much higher thrust and therefore, on aircraft with wing mounted engines like the 737, there is a significant swing (yaw) encountered which must be controlled with rudder input.
On approach the engines are providing less thrust so any failure of one won't induce as much yaw. Also, an aircraft finds it an awful lot easier to descend on one engine than climb
Both are manageable though and we frequently get them on simulator checks.
Hope this helps
