Either can be used, depending on your workflow.  Live re-renders allow for realtime monitoring and recording of them via Dolby Renderer Return plug-ins in Pro Tools, whereas offline re-renders are made faster than realtime direct to disk.  Offline re-renders can then be imported into a DAW timeline if required.  While each offline re-render is created faster than realtime ones, depending on how many re-renders you are creating and your overall CPU load will determine which process is quicker, as you can generate up to 10 re-renders in realtime using the live re-renders.