Absolute Gravimetry widely uses optical interferometry methods to track the trajectory of a free-falling body. Processing the space-time coordinates recorded during the flight, the local gravity field g is evaluated. Since the g value affects only the vertical motion, a Michelson interferometer is settled with one of its arms perpendicular to the ground. Any deflection from the vertical direction introduces an error on the recorded trajectory. The assessment of the bias and the relative uncertainty due to this phenomenon is reported. The analysis, referred to the IMGC gravimeter that adopts the symmetrical free-fall method [1], applies also to other methods (simple free-fall).