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For me it's when the game shows any weird behaviour (wrong sprites being drawn, graphical glitches, wrong sounds being played, errors that don't make sense, etc...), and ALWAYS before building an executable, especially if I'm building using the YYC. If you're making a game for HTML5, I would definitely recommend clearing it before building a test after adding any new GFX or SFX as the HTML5 target seems to be much more sensitive to changes in the cache.
This is more related to "per session" maintenance than between builds - but make sure that the tickboxes for all 3 Automatically delete * cache directory on close in the IDE preferences are ticked (can be found in File > Preferences > General > Paths)
This makes sure that every time you close the IDE the caches are automatically deleted, which will prevent you ending up with possibly gigabytes of old builds and cached files taking up hard drive space and means you never have to worry about it again.