I'm going to disagree a little bit with Charlie, just a little bit. What I've seen is that the low end stuff plays two roles, one is that it's out of the way of the dialog, and two it's going to fall off faster then the high stuff when turned down. So I find, obviously depending on the cue etc, that I tend to add more of it IF I have re-recorders that are turning the music down. I guess it's part of how the ear hears that the loudness curve is not linear.
But there's another aspect to this issue- the idea of the density of the music against the picture. Sometimes you're just f&$ked and the scene calls for lots of noise from you, but you KNOW it'll all get turned down. The effect of loud music turned down is a peculiar thing for me, it kind of runs counter to it's original intention, that is, to be heard freakin loud. It is the ONE thing that I think about more then anything else when starting to write a cue- namely the density I'm after. Frankly I think any experienced composer is going to be sensitive to that first and foremost.
One thing I've noticed in some mix rooms is there are level control plugins strapped across the music subs somewhere, sometimes acting like duckers, or sometimes just for level police. I think at the end of the day it doesn't matter how you engineered the score, between the mixers and the producers, fader levels become the first line of defence when the assault of sound coming from the speakers overwhelms the output. In this case, working to a lower printed level can actually get your score across more as you intended. And it gives the mixer more play room with the fader because otherwise he's got it down into the -25 range if you deliver fully compressed, up to 0dbfs cd master level stuff.( not that they haven't seen that, not does it phase them either way )
Frankly, some sfx guys are amazing at being "lyrical" in their sound design, while others, in my experience, throw alot of noise at the screen (imo, too many layers of noisy sfx). I've heard many a section come back with completely unmusical sfx overtop a section that buries the score and the first thing I thought was "if I stripped away 80 percent of that shit, we'd have a scene that is starting to connect" emotionally.
Well at the end of the day, if the call is for over the top music, you just do it and make it as best you can and let someone else move their little finger and lower the volume of your opus to the point you barely hear it above the "ambience" cut in while h-bombs are going off left right and center.
sigh.