I have used the following policy on a wide variety of foreign and domestic bike tours with great success: Don't be paranoid. Put a reasonable faith in the honesty and good nature of most people, but use your head.
In small rural towns, I often feel completely safe leaving the bike unlocked, but I do keep my camera, passport, and wallet with me. In bigger towns, I use a cable lock. I know this approch won't make me completely safe, but I don't expect life to be completely safe. Worst case, I might have to buy another bike someday, which wouldn't be the worst thing in the world. In 30 years of riding, I've never had anything stolen.
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"Light it up, Popo." --Levi Leipheimer