I have a disc lock that I use on occasion, but 95%+ of the time, I don't find it necessary as I generally ride out in the sticks and not near very many people. It's not uncommon for half of the guys I ride with to roll up to a restaurant for lunch and leave the keys in the ignition, helmet and jacket on the bike, and tank bag in place. I've even left my wallet in the tank bag once on accident. Sure, it's not the best idea in general, but being aware of your surrounds is a good way to judge whether the bike is safe. Generally, if I don't feel safe to leave my helmet laying on the ground next to the bike (so it doesn't fall), I'm not going to leave the bike.
If I was traveling any serious differences, or overseas, obviously I would feel different and prepare as needed.