Hi KarenT,
"RC" stands for "release candidates", so recent code released by an RC may be broken. People using RC's should report any bugs using the bug tracker. As RC's may introduce new features, they also may serve as a step-by-step transition (RC1 to RC4, for example) to the most recent release that will come eventually (1.
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So, for my own programs I can say that transitioning from 1.6 to RC3 and RC4 has been smooth.