Just to be awkward...
I'd advise against adding any superfluous information to your user agent string. Ultimately the only thing web developers need to know is the name and version of the rendering engine your browser's using, so they can serve you a page that'll be displayed correctly. The aim is to make it as difficult as possible for attackers to footprint your machine - by revealing the operating system you're using, it allows them to make assumptions about other software installed on your machine. Granted, on Slackware it's more difficult because it's unlikely that two installations will be identical, but leaking as little information as possible is a good habit to get into, in the same way that portscanning your own machine and disabling unused services is a good habit to get into.
Incidentally, this is one reason I don't like the .NET Framework browser extension on Windows, because it does things like this to the user agent string (this example taken from What's My User Agent?
five minutes ago):
Mozilla/4.0 (compatible; MSIE 7.0; Windows NT 5.1; InfoPath.2; .NET CLR 2.0.50727; .NET CLR 3.0.04506.648; .NET CLR 3.5.21022; .NET CLR 1.1.4322) 2011-10-16 20:22:33
Instantly you know that this person's using a beta version of the .NET Framework 1.1, which is vulnerable to a whole host of attacks
. (I think the JRE used to modify the user agent string too - thankfully it doesn't any more.)
Don't make their lives easy.