It can make a miniscule difference in sound. It used to be important with woofers with a clothe surround that could be "stretched" with today materials, only the geekiest of audiofools need to break in their speakers. Plus, this only really helps if you are building your own speakers. Breaking in causes a small variation in the T/S parameters in most cases. If you want to follow through on the small benefits, you would need to remeasure them.
