A nice new feature coming in Dreamweaver CS 3 (out in April from Adobe) is the ability to drag and drop styles in the CSS styles panel to change the order of the styles in the style sheet. It works in the same way moving a layer up or down in a Photoshop document works. Just grab the style you want to move. As you drag it a black bar appear that indicates where it will drop.
In my latest book, Mastering Integrated HTML and CSS I have a chapter that explains how to use CSS with Dreamweaver. At the time I wrote the chapter, the only way to reorder styles in a Dreamweaver style sheet was to open the style sheet up and cut and paste to achieve the order you wanted. If you own that book, take note of this new feature, because it makes keeping styles in a particular order for cascade purposes much easier.