The general rule for perennials is that things that bloom in the spring are best planted in fall, while those that bloom in summer are best planted in summer. A good example of plants that do best with fall planting are peonies. But many plant them in spring and they do fine.
It would be harder to locate summer blooming perennials in the fall to try planting them then. I actually have no experience of doing this so I am not sure what would happen if one tried fall planting of summer bloomers. In spring, the nurseries seem full of both spring and summer blooming perennials, so from that perspective it makes a lot of sense to prepare your beds now and wait for spring.
I've not tried the newspaper/cardboard thing, but even if you found in the spring that there were still pieces of paper that had not broken down, you could simply discard them as you find them.