By themselves, rain pants aren't ideal to hike in. While they often have some measure of breathability, it's not enough to keep you from getting sticky and sweaty while you're on the trail. At the same time, putting them on after it starts to rain partially defeats the purpose of bringing them in the first place. At the end of the day, it's a judgement call based on how well you can track the weather. Ideally, you'll put them on right before it starts to rain.