IMHO, the current best practice is to avoid it if at all possible :-)
By design you data could be coming from anywhere, at any point in time (replication) so unique constraints are not really practical, even with the _id field.
Having said that, couchrest_model's validates_uniqueness_of should work for most simple cases where there is a low probability of collision (two users with the same email address is not likely.)
Its not documented yet, but you can also do "scoped" validations:
validates_uniqueness_of :title, :scope => :parent_id
This would use the view "by_parent_id_and_title" to check for existing matches.
Hope that helps,