TotWL never fills to capacity. Maybe on the 4th of July and/or New Years they might...MIGHT.
In most cases, if all you are doing is going to see the fireworks, I would get to the elevator at the bottom of BLT about 15 minutes before showtime. This will give you enough time to get up, and get a spot to watch.
If you want a few drinks, i'd recommend getting up there 30 minutes before showtime. A lot of people show up at once, and there is only two people up there serving drinks, so it can take some time to get served during that time.
But if you do go up there... PLEASE grab a drink or two. The lounge is there to make Disney money, and if they aren't making money off of it then they will close it. Then we won't have it as an option for fireworks viewing anymore