Personally, I prefer a lot of rules just so I can meet the other participants expectations. However, for a group used to working together I think the pushing the rules people/rounds are often the most successful.
I'll have to look for them, but I used to have a link to a site that had several successful strategies.
Bkay, typically you have a small (maybe 6) group of people and everyone makes their own center, then it gets sent to another person to add a border, then to the next. Typically it is a one month turnaround. Sometimes the rules are that Round 1, Center, needs to be made so it can be on point. Then Round 2 is to add triangles to put it flat. Round 3 might be to make the shape rectangular (I see a lot of square quits in round robins) by just top and bottom borders. Sometimes a round requires a technique, like applique... those are the sorts of things Western Wilson are looking for.
There are many variations, sometimes everyone follow a certain order, that is you always get the round from one person and always send to another. Sometimes it is designed so you never get/send to the same person. Sometimes people include fabric that must be used or can be used. Or specific styles like modern or primitive or the list is endless! It takes lots of trust to do a round robin (especially by mail and not as a guild or work group project) and you have to be open about what you receive back especially in an "anything goes" style.