Gaucho, it depends on the tariff.

The classical rule (e.g. the SQ/AC RTW) is strictly no backtracking. Once you start eastbound, no westbound at all. But some fares allow for exceptions (Like YUL/NYC on some AC RTW's. or BKK-HKG to originate a CX/UA out of Thailand).

It is only the newer, milage limited fares, like the OWE, the RWSTARS, etc. that have the more flexible rules.