The only time I use px measurements are when I'm trying to limit the maximum size of an object. For example, an older book might have small images. I want the image to fill the screen as much as possible without getting over-stretched and blurry. In that case I would set the image width to be 85% of the screen, with a maximum width of the original image width.
Code:
ExampleImage.jpg = 500 x 800 pixels (w x h)
div {margin:2em auto; width:85%}
img {width:100%, max-width:500px}
<div><img alt="" src="../Images/ExampleImage.jpg" /></div>
Other than that, I would want to use measurements that keep everything relative to the user selected font/margin sizes - like 'em'.