I only took a first quick look at Dreamweaver last weekend, so could you help me along as to what "planning the site" entails? I'd say CSS comes into play when you reach the stage of laying out your pages and deciding on looks and such; i got the impression Dreamweaver leans strongly on using templates, so when creating a template you definitely have to dive into CSS as well.
You have answered the question, seeing as you would have to plan what your site will have to entail in order to create a .css file to allow you to make sure that you know what you want to format with the .css file.
If you need anyhelp let me know, and I will see what I can do.
Content and functionality is (and should be) developed entirely independently of presentation. html (including css) is irrelevant for content. But when you start thinking about presentation, you might as well do it 'properly' right from the start.
Yes - then you have a design template based on the actual content of your pages.
More specifically, I'm talking about using the best HTML logic you can before looking at CSS. For example:
<p>Welcome to my website. <b>I hope you like it</b></p>
Now that's coherent in any browser, all the way down to text-only and legacy browsers. Working with a text-only browser is actually very useful - it helps you to get an idea of how a page will appear to a SE or a speaking-browser.
Once you have that, you don't need to worry about it anymore. Go wild with CSS safe in the knowledge that accessibility and degradability is all but taken care of.