I really don't think culling any pumpkins is a necessary practice for anyone but competitive AG growers. That being said female flowers that bloom after a month before your first frost won't stand a chance. I see you are in zone 3, I'd need more info to look up your frost dates, but say it is Sept. 15th anything that tries to form after Aug. 15th won't stand a chance. Again that was just an example and you should look up your frost dates. The pumpkins should be picked before a heavy frost, keeping in mind that pumpkins that are dark green and showing any signs of changing color will continue to do so once picked. In zone 3 I'd imagine most of your pumpkins will be green and ripen off the vine.