By combining Raman scattering from the cleaved edge and under hydrostatic pressure, we have accurately determined the tetragonal phonon deformation potentials of strained Si1-xGex alloys in the entire compositional range for the Ge-like, Si-like, and mixed Si-Ge optical modes. A known biaxial strain is induced on thin alloy layers by pseudomorphic epitaxial growth on silicon and subsequent capping. We also determine the strain shift coefficient of the three modes, which are essentially independent of Ge content between 0.4 and 1. This is key information for an effective use of Raman scattering as strain-characterization tool in SiGe nanostructures.