To always start in theater mode go into Options then click Startup. Near the top of the of the screen under Mode: select Theater View
One thing that annoys me when opening directly in Theater View (whether via arguments in a shortcut or by changing the option) is that when you exit Theater View, Standard View remains open.
If I open in Theater View directly, Standard View should
not also open in the background for me to have to grab the keyboard to close (I like leaving my HTPC on the desktop as I have a 10 second rolling background). Theater View should almost be a "separate" application (.exe) that you can launch without having to have Standard View open too (then you can create a shortcut to the Theater View .exe instead of using arguments).
Or maybe, instead of faffing about with all that, have another Exit option in Theater View: Return to Standard View. If you choose this option, Standard View then remains open. If you just Exit Theater View, Standard View should also close, even if you previously chose Return to Standard View.