Pros are often feeding signal to other hardware equipment. In this scenario headroom is important.
If you are staying in the digital realm, you are a little safer going up a bit on the levels.
Besides headroom there is signal-to-noise. Many hobbyists focus on this aspect. Numerous tracks add noise and if you record a strong signal you can lower signal-to-noise, but at the cost of headroom.
I'd worry more about whether I was using line levels that are consumer (-10) or pro (+4). That may also come into play for the reason 'pros' say -12; they are likely using signals with more energy than hobbyists are to begin with.
Here, maybe this will help-

http://www.independentrecording.net/irn/columns/jwal/index.php?id=79

Make your sound your own!
.. I do not work here, but the benefits are still awesome