Well, some applications don't have any title bar at all: especially audio or video applications, I agree. Or they have their own native one.
But, if you take VideoLan for instance (
www.videolan.org), the standard interface (i.e. a "normal" window) shows the system menu when right-clicking on the title. While an interface with a theme for the same program doesn't.
Firefox and Thunderbird have also completely modified the whole title area, to give more place to the web content.
So, my interpretation is:
- if you have a "standard" window, you should get the "standard" system menu when right-clicking on its title.
- if not, feel free to do as you wish.