Once you have installed your site, it is true that you are not required to complete any configuration to start collecting inventory. For software inventory, though, you will need to define some rules to start collecting data. Without any configuration, inventory will work and collect the default set of inventory from your clients.
The implementation of any feature is as important to the success of the deployment as the design of the hierarchy. For this reason, you should always plan and design each aspect of your configuration; inventory is no exception to this.