The logon dialog can be disabled by using the NoConnectionList command-line parameter. For example:
"C:\Program Files\PLSQL Developer\plsqldev.exe" NoConnectionList
You can also set this in the params.ini file in the PL/SQL Developer installation directory.
The Connection List in PL/SQL Developer can be docked or closed. If you subsequently select the "Save Layout" item from the Window menu, this will be the default layout.