There is a theory floating around that says to link with the full domain to remove the questionable ability for the canonical hostname to be duped and make sure that all references in google refer to
www.domain.com rather than some being domain.com, some being
www.domain.com. Google's 'dupe' page detector seems to have gone a little wacky recently and has a problem with the ww.,
www., wwww. etc that some servers support.
However, I hate using full urls in pages and will never do otherwise. I do almost always do an absolute anchor, i.e.
<a href="/dirname/">
rather than
<a href="dirname/"> or <a href="../dirname/">
and if it is a directory, I always use the trailing /, so, I never do <a href="/dirname">
Although, if your server was configured correctly, <a href="/dirname"> would indeed redirect the person with a 301 redirect to the 'servername' -- presumably eliminating the canonical name issue altogether.