At this point, I think it's down to whatever makes the most sense to you. You can always try something different later.
I have a "watch TV" command that does a few things with my amp, lighting, and launches XBMC. I have a "listen to music" command that does some other things with my amp, and launches media monkey. You could do something similar if that works for you, and in those commands you could set the default audio appropriately.
I'm not sure about iTunes but media monkey lets you opt to use either the default audio, or a specific audio output. XBMC Eden lets you use the default audio, but XBMC frodo does not, so there is no easy way to switch audio output with frodo. So if you use MediaMonkey you can have it always use the analog outputs. MediaMonkey is a great choice for music I think, but there is a bit of a learning curve vs. ITunes, and no 10 foot interface as with XBMC. I find with music, I don't need this, and prefer voice commands anyway, most of the time.