There aren't any guarantees either way. It's always a gamble. There may be hidden internal rotting going on that you can't see. I've picked and cut open pumpkins right from the vine and found them with black mold or rot inside.

Cool conditions help them to last longer -- if they do start to rot, the microbes grow more slowly. A little rain doesn't seem to make much difference one way or the other. The more likely danger in wet conditions is to have them sitting on something constantly wet, which encourages not only mold, but also slugs that like to take bites out of the bottom of them, which then promotes rot in the damaged spots.

Don't leave them where they will frost or freeze, that will eventually destroy them if cold enough.