No offense, but putting critical financial data like invoices into a schema-less datastore strikes me as short-sighted and risky in the extreme. Best case, you're making a bunch of extra work for your accountant; worst case, you could completely fail to impose the structure needed to do basic reporting and billing. Financial data is highly structured already; I'm not sure why you'd throw away all that nice regularity and structure in order to save a few inner queries.

Also, in the case of a simple blog app, I don't think that contention is going to be your biggest worry. The on-disk structures for CouchDB are append-only, so your biggest worry isn't going to be locking, it's going to be the stale document revisions taking up disk space between vacuum operations, and the replication overhead for all the intermediate versions.