You do have to specify the PIE behavior for each CSS element that needs the PIE help, but you can use one rule to do it. For example, if you have 3 classed divs and every image in the main wrapper that need PIE's assistance to render CSS3 elements in IE8:
div.callout, div.special, div.best, #wrapper img { behavior: url(/path/to/PIE.htc); }