Trim at any node where leaves or roots come out. You can trim only the roots off the stem as close to where they come out of each stem but they will continue to grow back. Those roots will last forever until they are cut off or die off. One reason these might be growing roots like this is because you planted so many stems close together and now this one stem is reach for more nutrients. Always spread you stems out by 1\2" at least when planting them. And for your last question the answer is no. Don't trim and replant stems just because you see these roots. You trim plants to propagate, clean them up, shape them, or remove them.