Well, good question. The honest answer is I read a suggestion about it online. So, thats the way I did it.
Now that you've asked I have occasion to analyze the technique.
I doubt it contributed to the sharpness of the lines because 1) i wasn't using a long telephoto, 2) vibrations from the mirror would occur only during the fist part of the exposure, 3) i suppose the variations in the lines caused by the fireworks themselves would mask any blur caused by the vibrations
So... next time I suppose I will try it w/o mirror lock up!?
Ed.