I weight in about the same as Txulrich and back my ring buckles up with a slip knot as well. Even if my weight weren't a factor, I'd probably go ahead and back it up anyway. I really don't see a reason not to, better same than sorry, IMO. When I was in service, one of the things they taught us was that anytime a knot supports any significant weight, especially your body weight, back it up with another knot.
"If you play a Nicleback song backwards, you'll hear messages from the devil. Even worse, if you play it forward, you'll hear Nickleback." - Dave Grohl