I traveled overseas for 3 straight years long ago, and with the exception of when I had my own locked apartment, I kept my passport physically on my body at all times. The comment above about developed countries is provocative. I know very little about Argentina, but I believe in the 1930's it was considered on par with European countries and North America in terms of development.
London has some pretty effective pickpockets (try Covent Garden on a busy weekend in Summer). Prague petty criminals prefer the accidental bumping into you approach, especially in the main tourist area outside Tyn cathedral/Astronomical Clock in the old town square.
I must admit that I relax a little when I enter an academic environment.
I have one of those wallets that have little straps that hook over your belt, and then you put the wallet inside your trousers. Works for me.
