Painting on stretched canvas on stretcher bars is not recommended. It is not rigid and when heated and covered with wax it sags. It is also important to note that most canvases from an art supply store has been primed with acrylic gesso, which is not compatible with encaustic.
If you want to paint on canvas or linen, first stretch it over a ridged support. Ideal surfaces for the encaustic artwork are absorbent, rigid and heat resistant.