To prevent one from inadvertently feeding the spammers' feedback loop, in Mail's Viewer Window I have the dividing bar pulled all the way down so that mail can only be opened in a new window. Thus, if I think it might be spam, and I want to check without releasing the little spies (requests for associated HTML files), I hit reply (or option double-click) instead of double-clicking to open. This gives me a chance to look at the text content without the html files (when opening replies, Mail doesn't go out and look for the html files as opening the original does).
[robg adds: An earlier hint discussed moving the preview pane down to avoid sending spam tracking tags back to the source ... but opening a reply email to check the content of a questionable email is a new one to me (mainly because I just disable HTML globally in Mail and then use the preview pane).]
Mac OS X Hints
http://hints.macworld.com/article.php?story=20040220081414214