Is using negative values in CSS acceptable?

Let me consolidate my comments into an answer here.

First: Negative values are perfectly acceptable however I would use them sparingly. They are definitely useful in certain situations

HTML markup should follow the visual flow of your page. I wouldn't use a negative margin in the way you originally intended. One common use is when overlaying elements. Ex:

How would I fix your top menu code? I would wrap #status and your UL within #topnav like this...

<div id="topnav">
  <div id="status">Logged in</div>

And use something along these lines for the CSS...

#topnav {
    float: right;
#topnav #status {
    text-align: center;
    margin-bottom: 15px;
#topnav li {
    display: inline;
    margin-left: 15px;

Here is the demo:

Hope this helps!

Negative values can be very powerful when used correctly. There is no problem with using negative values.