When describing the steps for backing up a sharded cluster, we mention that turning off one of the config servers prior to a mongodump is the best practice.

In fact there isn't really a need to shutdown a config server as long as the balancer is disabled. Turning on and off of config servers can cause other issues in the cluster regarding balancing and availability.

Currently i can find this being stated at http://docs.mongodb.org/manual/tutorial/backup-sharded-cluster-with-filesystem-snapshots/

But I suspect it may be rolling around in multiple places as well.