Does the ‘VIRUS CHEST’ need to be emptied?
Not, not really. But if you want to recover the files space used, I suggest you rescan the files within chest (right click them), and check if you don’t need them anymore. Then delete 
Clean, Quarantine, or Delete?
http://antivirus.about.com/b/2007/03/11/clean-quarantine-or-delete.htm
avast! 7.x: Using the Virus Chest
https://support.avast.com/index.php?_m=knowledgebase&_a=viewarticle&kbarticleid=1406