For most .40 size and smaller planes, the stab shape is really for appearance only.

Larger planes, especially Pattern competition models and others expecting high loads on large stabilizer surfaces do have the strucural/weight advantages of the sweep/taper as a major consideration.

At the relatively low maximum speed of models, we practically never need to consider Mach effects. The wings and tailplanes just aren't goig to start creating a Mach buffeting effect at the models size and airspeed. ( a major reason for sweeping wings/tailplanes of 400+ mph full scale aircraft) A model aircraft's prop though... can have the tips exceed .6 Mach, and that can create a LOT of noise. (that noise is wasted power...)