Virtually all the mining pools I have seen specify a few ports with differing initial difficulties. The difficulty adjusts automatically according to your hash power. I notice that monerohash now offers the ability to set a custom fixed difficulty at a minimum 50000.
What are the benefits of locking down your difficulty? In what circumstances would it be best to opt to specify a fixed difficulty at this value or higher? Would it ever make sense to set it to be lower (hypothetically)?