A tapered trailing edge has less drag so if speed and efficiency is important then taper away. If it's for a general sport model and uses a big engine then you can safely leave them square and full thickness. At least on the tail.

On the wing leaving the trailing edge too thick and square will deform the airfoil over the last part and may cause some problems in slow flight. If and how much damage is done will depend on the airfoil and how you expect the plane to fly. If it's light enough then you can almost get away with murder of the aerodynamic sort much like the 3D flat foamies do. But if this is a heavier model then it's probably wise to put in the little bit of extra work at least on the wing.