I have fairly pricey outdoor cushions (Sunbrella canvas linen from Resto. Hardware). I leave them on the outdoor furniture all summer, but all the furniture gets covered when not in use - they are treated to be "water-resistant", but if caught in a downpour will still take a long time to dry. The cushions get cleaned and stored in large waterproof cushion bags in the garage during the winter. If I had the cheaper more plasticy type of cushions that are more water resistant, I might leave the furniture uncovered more often during the summer, but it's really not much of a hassle to pull the covers on and off.
I made the mistake of not bringing in the cushions on the front porch swing this winter since the porch is covered and they're pretty sheltered, and now I am going to have to try and clean squirrel pee off of them.
If you do store them inside for the winter, make sure to clean them off thoroughly first...I missed a spider egg sac on one once, and when I opened the cushion bag the following spring, there were hundreds of little spiders inside!
