It is fine to heat,  moldable liners, modern liners were all designed in some matter to be molded.  Now a normal liner will break in the same as heat molding in about 2 days, and can take a few heatings before they begin to break down.  Now intuition liners that are found in dalbello and full tilts, were designed to be heat molded in a convection oven, and can be molded two times before they begin to shortlast.