The Friday night for which we were there this year HAPPENED to be the 4th of July, as fireworks shows go. Awesome fireworks show, by the way. As good a display as any I've seen in many years.
Having been to HHV in May 2006 (Rainbow Tower, not Lagoon) and seen the standard Friday night fireworks at the time (which was still good), we actually asked for a room this year on the Diamond Head side of Lagoon tower, thinking we would have the best seat in the house for the fireworks on the 4th of July.
As it worked out, the fireworks show on July 4th was unfortunately over the marina, which put our unit on the back side of the tower from the show (and which put us on beach towels next to the lagoon for the show). Easy come, easy go.
Bottom line (at last!): If the normal Friday night fireworks show is still as it was back in May '06 (out over the ocean between Rainbow and Lagoon Towers), you may be good from either side of Lagoon Tower, but most certainly from the Diamond Head side. If the Friday night show is now over the marina (as it was on July 4th), then the marina side it'll have to be.
After all my rambling, I'm not even sure I answered your question.

I hope it helps.