I don't think there is a set list but I will give a list of places I would recommend only sending registered to.
All of Africa, south america (I have sent to argentina and brazil unregistered with no problems so far), russia, italy, mexico, india, middle east and eastern asia (except japan and china). Europe, Canada, US and Australia are generally pretty safe to send unregistered.
Forgot to mention that I have traded with someone in Singapore and it is safe there as well. I have sent unregistered to Italy and no problems with my letter getting there. But the guy I swapped with sent registered to me and his letter arrived to me empty. I think that sometimes sending registered makes you a target.