It is very rare that WDW cancels fireworks due to inclement weather and, even then, it would be at their discretion to offer you a refund or not as they do not guarantee fireworks as part of the event. We always know we are taking a risk when we book the MK party and I always set a reminder on my phone to cancel 5 days prior if we are on the fence about going. I've done the party 5 times and its never been canceled even after a huge downpour one August afternoon. We've even watched it in light rain. They really, truly do their best to do the fireworks every night even if it means postponing them a bit (we once watched Illuminations after they delayed the show for an hour due to heavy rain and lightning). If you book it, I would just be prepared to cancel within the given timeframe if you decide not to go or, you could always try to book it closer to the event, but you, of course, run the risk of having no availability at that point.
