Using finite-elements we study the postglacial rebound in an axisymmetric Earth with a non-Newtonian rheology. Our approach allows for the first time a self-consistent study of the effect of a power-law creep on the long-wavelength deformations, which cannot be performed using flat models. We show that, for a non-Newtonian upper mantle, the time-variations of the low-degree Stokes coefficients become largely insensitive to the viscosity of the lower mantle. This is in contrast with previous evidence based on Newtonian models, which emphasized the importance of the low degree signatures for constraining the lower mantle viscosity.