Guild Tasks are not meant to be guildleves where you have to craft required items in order to complete the task. All they ask for is to deliver certain items to the guilds and you are not forced to synth 'em by yourself. I mean, it's not a case of "We need Culinarians for bringing us Sea Sands"; it's more a case of "We need Adventurers for bringing us Sea Sands", so I don't see the point of placing Guild Tasks' rank requeriment in order to complete the tasks.
Just my opinion, though