It is first come, first serve.
(Note: the stuff following consists of my views, not necessarily IdentityUnknown's)
I disagree, but not because I don't want everyone to play, first of all. The reason is the complexity of doing so. First, how do we decide who gets to play and who doesn't? By the number of rounds they participated in? What if all those rounds were not next to each other? What if that round in particular was lackluster? How, indeed, do we decide?
Even if we manage to decide a basic set of factors, the list would have to be updated frequently. Also, how many people should be given this special privilege? Say 8, the current established guideline. If all 8 people decided to invoke their power, they would effectively 'block' off a round not only for regulars, but for other fairly new people. If the number is too low, not only will not enough people be satisified, but the list would have to be updated more frequently than usual. I personally believe that there should be no reservations made for DCW IRL rounds under all circumstances (in fact, I've requested IU about this), but we already have MVRP. Do we really need more reserved spots? For every reserved spot, another person cannot join.
My ultimate belief is that if you want to play, sign up quickly. And if you don't, it's tough luck. That's how life works, no?