I have done it both ways, just ironing the edges then shrinking with a heat gun, and ironing the whole thing down to the solid balsa. Use a scrap chunk of balsa and see what works best for you. Always good to practice first anyway.

The Mustang in the picture I did with the heat gun method. If its a flat surface and you have stretched it out good while tacking the edges then there is not much air under the film to expand anyway.