It depends on what your Mac does when you close the lid. It will usually give OmniFocus a little more processing time, but if it immediately drops its network connection processing time won’t help since OmniFocus won’t be able to reach the sync server.
If this is something you do frequently, you might want to get into the habit of pressing Command-S first to save and sync your edits, or you might want to change the default sync settings so OmniFocus syncs within 15 seconds rather than one minute.
