Unlike Mail, Spamassassin assigns a spamminess value to each message. I decided to turn these values into colors, in order to be able to peruse my junk folder for false positives more easily. It works like this.
Open the Preferences in the Mail application, and select Rules. Then, add rules that look for the X-Spam-Level header. This header isn't in the default list, so you'll have to add it first using "Edit header list...". Select "Begins with" and then add a number of *'s. I use five for my first rule. Then under actions, choose "Set color" "of background" to something that indicates a low level of spamminess, such as yellow. Then add additional lines with more asterisks and more severe colors. I use orange for ten asterisks and red for fifteen. Make sure that the rules that match more *'s come later in the rule list.
The result is that anything that probably isn't spam doesn't receive a special background color. Anything with a score of five or higher is highlighted in yellow. Most false positives are in this category, so I open the message if there is any doubt whether the message may be legitimate. A score of 10 or higher means orange highlighting, and 15 or higher red. There are very few false positives (legitimate messages flagged as spam) in these categories, so I just look at the subject and sender and only when I'm pretty sure it's a legitimate message I open it. No benefit of the doubt in these categories.
Note that Mail will also sort messages based on color (under View, Sort by). This makes scanning the junk mail folder for false positives even easier.

