The deal with official RTW fares, at least in 2000-2008 when I was checking them, was to buy them ex-interesting country vs. ex-USA. They have a price set in local currency, so when a currency drops vs. the USD, it takes them 6-12 months to adjust prices. ex-CAI, ex-BKK, ex-KUL were all excellent places to buy for a long time.
A guy on flyertalk would maintain a list of ex-various *A RTW prices. https://docs.google.com/spreadsheet/ccc?key=0AuTBktAWbGhwdHd...