There is a concept of a stable timestamp implemented in
WT-3387. The stable timestamp is a timestamp behind which it is safe for WiredTiger to make content durable on disk - which means that it becomes OK for WiredTiger to trigger reconciliation of pages with updates behind the stable timestamp. It is also the most recent timestamp it is safe for MongoDB to create a checkpoint at, though I don't believe we need to enforce that in WiredTiger itself.
The stable timestamp can be greater than or equal to the oldest configured read timestamp.