There isn't any harm in leaving the plant stalks in place, although this encourages rampant self-seeding, which is generally not wanted. You should not mulch over the root crowns, as This will promote rot. To ensure you’re planting milkweed that should help your local ecosystem and draw in native pollinators, https://mylesxzzyy.iyublog.com/30335489/a-review-of-is-butterfly-milkweed-a-perennial