Will auto_increment always grow in master-master setup?
Auto Increment ID is "replicating" to the slaves. Otherwise it would be hell of a job to promote a slave to a new master if the status of auto increment needs to synced. So to answer your question it would start from 8. 

If you do not write actively to both master at the same time you don't need to do any auto_increment_* settings trickery. These settings are meant to avoid situation where the two master picks the same ID for new rows inserted concurrently on both masters which would lead to conflicting IDs while applying changes from the relaylog.