I don't have a "for sure" answer, but when we were there in November the bus driver was announcing the new Wishes show and said it would be shown that night at whatever time (7:00PM I think - parks closed early) REGARDLESS of the weather.
When we were at WDW the first week of february 2004, there was NO Wishes-fireworks because of the weather, it rained too heavily. Sorry to disappoint you, but they do cancel it now and then.
We worried about this one night on on Jan. trip. It had been raining steadily throughout the evening - not pouring or a thunderstorm though. I asked one of the CM's about it and they said that it *can* close due to rain, although they were pretty sure it wouldn't that night. Sure enough, Wishes was on that night. No Tink however, and a few other nights that week when it was really cold Tink was "resting" HTH!
We were there a couple weeks ago, and on our last day it rained a little bit, but we were concerned because we wanted to see fantasmic and wishes, so I asked a CM about the weather. He told me that fantasmic will usually only be cancelled for thunderstormss, but wishes would be cancelled for heavier rain showers.
While it might not be cancelled due to weather, the rain can cause problems for viewing the fireworks. We were there in February and it was misting. You actually couldn't see the fireworks! The only ones you could see were the white ones that go sideways (that's the only way I know how to explain it, LOL!). The different colored ones around the castle were blocked by the haze and mist.
I wouldn't worry though, I think it has to be a pretty good rain for them to cancel the show. In mid-March it was definitely raining hard - not quite pouring but just about there, and the show went on. We still got some great pictures, although I was very upset that there was no Tinkerbelle.
When we were there back in December, the show was delayed for 40 minutes! I was glad the show wasn't canceled as this was our only planned opportunity to see it.