In Navisworks 2013 you can now append rvt files straightaway. Your rvt files still get converted to a nwc file. Bear in mind the following if the rvt file is of a lower version than 2013 the file get's updated to 2013 first and next it's being converted to a nwc file
Important: the original rvt file is not upgraded!!! Don't worry! I am merely writing this to let you know that the conversion process will take a long time if you are appending big rvt files. You might want to consider creating the nwc file in revit if you are using a older version than 2013 release.
Also you want to create a 3D view in revit called Naviswork. If you append rvt files you can set the rvt file reader in Navisworks to search for this view and only export those elements that are visible in this view.
Notice that my export settings are set to export in feet. Of course I work in the metrics system but Revit itself runs on an imperial engine... If you set it to millimeter or meter your properties / quantities will be wrong.
Perhaps sometime we should let Autodesk know that the majority of the revit users work with the metric system and they do not want their units converted to imperial.