I too like this thread and how nicely your plants are growing Tim. I agree with Paul it is preference.
Tim you and I have similar weather in the winter - very wet, so I do have to cover mine or they rot, I tried several different things but nothing else worked until I covered the whole area - this goes for all my semps though. I haven't found that they are any more prone to rot being mounded together or spaced, if you are getting enough rain/water they are going to begin to rot.
Your plants look great!