About "how trusting should I be", I would like to quickly bring up a bit of the scientific research. I've read a bit of this, and have concluded in high certainty that individual trust gives off a decent positive externality.
Quite a bit of econometric research (yea, it's not the best, but it's what we have) shows that positive levels of trust correlate with higher economic growth.
See:
http://oep.oxfordjournals.org/content/56/1/118.short
http://onlinelibrary.wiley.com/doi/10.1111/1468-0297.00609/abstract
However, I very much remember reading about how entrepreneurs have lower trust levels than most people on average, but can't seem to find the paper right now.
What I'd imagine is that in general trust helps society, which makes a lot of sense as it allows trade to function. However, it may very well be that the level of trust optimal for an individual is quite lower than that optimal for his or her society. Makes sense if trust is looked at similar to altruism.
In this case, obviously the more trust people have with other people and their bags, the more cases of stealing that will occur. On the whole this may help your surrounding people, as some of these will benefit from stealing. However, the disadvantage would come to yourself at the expected cost of having your stuff stolen.
It would be interesting to factor in not only how much your bags being lost would cost you, but how much they would benefit the person who steals them.
The point that's stuck with me from Fukuyama's book on this topic (http://www.amazon.com/Trust-Social-Virtues-Creation-Prosperity/dp/0684825252) is that there's not just one fungible bucket of trust -- the types of people and institutions that attract trust within a society tend to shape and limit the types of organizations that can be formed and sustained. He argues that what permits organizations to both scale (relatively) smoothly and then subsequently persist over multiple generations is the ability of essentially random people to form bonds of trust (as opposed to forming those bonds with family members or relying on the government).
I am just working on a list of rationalist rules I live by, and this is the one I have most confidence in, so it seems a good topic for my first ever post (which will be short as I have to be on a train in 15)
Since people routinely exaggerate risk, and social norms pull us towards the crabs in a bucket effect (especially for women) I want to correct for that. (Preferably without ending up with a giant Rob Me sign over my head, but that's not the direction I err in.)
For example, there was this rationalist walked into a bar. I had a lot of luggage - everything I need for a four day break, including over two thousand pounds worth of electronic devices and binoculars. I am insured, but it would be an especially annoying time to lose stuff. I had a coffee and then I needed the bathroom, which was far away through a lot of people.
I knew logically how little risk there was in leaving all my stuff; a Highland bar in the middle of the afternoon is even safer than where I live in Edinburgh, and no-one was pinging any alarm bells, but I still spent more time than I'd like to admit convincing myself I didn't have to drag the huge bag with me to the ladies and back. Yes brain, even though I'm alone, and the customers are men, and I'm a middle aged woman, and my mother would freak if she saw me...
Of course it was fine, like it was the last hundred times. One day I hope to not even have to persuade myself, but meanwhile I notice my prediction was correct and feel just a little bit pleased with myself.