I don't think you need a print event. All you need to do is adjust your @media print styles based on your Javascript(?) criteria. When the user attempts to print the page, the @media print style will apply and your styles will be in effect:
<html>
<head>
<style id="styles" type="text/css">
@media print { .noprint { display:none; } }
</style>
<script type="text/javascript">
var x = Math.random();
if (x > .5) {
var style = document.createElement('style');
style.type = 'text/css';
style.innerHTML = '@media print { .maybe_noprint { display:none; } }';
document.getElementsByTagName('head')[0].appendChild(style);
}
</script>
</head>
<body>
<div class="noprint">This will never print.</div>
<span class="maybe_noprint">This may print depending on the value of x.</span>
</body>
</html>
If you are using server-side criteria to determine what prints, then just have server-side code spit out @media print to decorate the classes as necessary. Also, you may want to consider modifying an existing class that's already inside @media print, or building up the new CSS using something other than innerHTML, which I'll admit smells awful to me, but seems to work in Opera 9.6, Safari for Windows 3.1.2, IE 6 and Firefox 2.0.0.17 (I didn't test any other browsers).