I hate to be nitpicky, but at level 62+, paladins have 7 or 8 auras (depending on spec) to go on the stance bar.
Ah. Never played a paladin, and didn't think about the fact that their auras go on the stance bar... I was thinking druids when I said six.
Anyway, even though the bar does size dynamically, I also always see a small "tab" (not quite the size of another button) on the far right of the stance bar that's unfilled, not sure if it block other UI elements or not though. I can test tonight and find out...
Strange... I don't see that on my druid, or my warrior. What do you get when you put the mouse over it and do:
It doesn't seem to interfere with other UI elements, at least for me, using r33676. I could ping the mini-map through it etc when the bars were locked.