It would be a nice feature to allow those logging-out, restarting, or shutting-down to request by check-box that the trash-bin first be emptied. (It would be innocuous to allow those putting the computer into suspension or hibernation to make the same request.) (Granted that the mindful or patterned user could always empty the trash before invoking the dialog.)
You can do this now by adding a script with the "Trigger" -> "on log out" to the session (in git master, will be part of Xfce4.14). https://git.xfce.org/xfce/xfce4-session/commit/?id=9e7e160d6db39d39283bef6fafb7264f4a65d9e5