We love stroller rain covers. I wouldn't go to WDW with out one. The 1st time we borrowed one from a friend last min and it save the trip. Now we had to get a new one for the double stroller.
Invest in one, $10, might not need it, it might be the best 10 dollars you ever spent!
If it might rain keep the stroller covered when you go in places, often you can go in a ride when it is not raining, and come out and it is dumping rain!