"Is it ever OK to take off when the aircraft is over gross weight?" - anonymous
The manufacturer has established a maximum gross weight for that airplane. This weight constitutes a limitation which must not be exceeded. It was established during testing by the manufacturer.
A pilot who exceeds this takeoff weight has no knowledge or assurance that the flight characteristics of the airplane will be acceptable. Additionally, FAR 91.9 specifically prohibits a person from operating an aircraft without complying with the operating limitations specified.
Information regarding the weight and balance for a particular airplane is located in the POH (Pilot’s Operating Handbook). The POH must be onboard the airplane where it can be used as a reference before flight, and to determine whether the proposed loading of the airplane meets the limitations described in the weight and balance section of the POH .
In the case of light airplanes, it is sometimes necessary to operate with less than a full fuel load to meet the weight limitations specified for the airplane.”