Templates are great if your networks are cookie cutter.
They're less good if you have slight variations - some config items can be overridden per site or by using device tags.
They shouldn't be used if you have significant variations.
The biggest benefit and limitation of templates (in my opinion) is firmware. All networks tied to the template must run the same firmware. Updating the firmware on the template applies it to all attached networks.
In terms of creating templates, you can create one at any time. It's only when you bind networks that they will change the config and potentially impact clients.
https://documentation.meraki.com/General_Administration/Templates_and_Config_Sync/Managing_Multiple_...
If the template limitations prevent you from effectively using them, get familiar with the API as you can more or less achieve the same result.