For the 'main show', or what is classified as prime time, I'd prefer a thread to opt in, rather than one to opt out. Which is what it essentially seems to boil down to. Very much so, in fact.

Forgetting to opt in is much less damaging (or consequential, or whatever...) for a writer, than forgetting to opt out. If you forget to opt out on a week/show you really can't go, you're basically losing by default unless you fire up contingency plans that might have a real life impact.
If you forget to opt in, well.... chances are you can still be added to the card last minute if you so desire, or you'll have to be happy with a character developmental piece and backstage segment, aka no harm done.

Just my opinion though, and I can imagine people disagree with me on that. But that's how I voted.