+1 for the original posters problem. I just made the switch from AVID at work and FCP 7 at home to premiere at home AND work, and looked everywhere for an answer (there isn't one) and this is a huge hassle.
I got in to trouble with this twice already when I did my assembly on the fly in a noisy place right after a shoot on a laptop and don't notice the channel assignments.
Then I sit down and to finish the cut I have to match frame every ******** clip back to the source, modify the source, and replace edit it back in to not have left hand dialogue for everything I recorded left channel only on my DSLR. Add a few dialogue edits to the equation, and you have a huge useless time suck.
Why they grey out the drop down to modify stereo tracks to mono when you access that menu from the timeline is beyond me. Shouldn't it stand to reason that I can change the audio assignment on my clips, tracks, or globally in the timeline like every other editing product?
I'm only a few weeks in to the software, but sloppy code like this that affects my workflow by 'greying out' simple effective options is enough to make me revisit that FCP X garbage.