As an 'opinionate' person (I hear you laughing...), I often find it hard to sit in meetings and not interrupt. So as a measure of whether or not I should speak up, I ask myself "Who needs to know that I know that?" and "Is it important that they know I know this? Now? Later?"
It has stopped me from interrupting many times. Not often enough...but more often. :-)
Recently, I attended an excellent discussion on leadership and storytelling - hosted by Rick Wolfe of PostStone. The point was made that one difference between a leader and someone aspiring to leadership is the switch from being the hero of one's own story, to offering the listener the chance to be the hero.
Interrupting and ensuring that the room knows that you already knew the fact/had the idea/like the idea/etc. is about you being the hero. A legitimate part of establishing brand and sometimes necessary thing to do (especially if the guys are not listening to the lone woman in the room as can happen). However, a good leader knows when it is better to let someone else be a star. Leadership isn't all about shining brighter; it's about encouraging others to shine more effectively.
The next time I'm tempted to interrupt, I'm going to remind myself that it's better leadership to sit quietly sometimes. My mentors are daring me to try...