Once passwword safe has opened the safe file and has been minimized to the tray bar, a Windows 7 computer will never go to sleep after the configured duration in the power settings.
To confirm that PasswordSafe is causing the problem, you can list requests that are preventing the computer from going to sleep:
I believe that once the safe has been locked, there is no reason that the computer should be prevented from going to sleep. This can leed to quick battery depletion on a laptop computer.
Other information: my safe file resides on a NAS accessed through a Windows share. I currently don't know if this is a factor for this issue.