This article is part of the Protecting WordPress index, but can be generally applied to your account and security practices!
Just like you complete daily chores around the house, you should regularly clean up your site and files that you do not need. Having old files on your account can leave you vulnerable, even if you've deactivated the old plugin or kept a backup of an old version in your web folder.
Stay clean and keep things organized - you should know all the files on your account well enough to identify when something is there that shouldn't be.