It is a must for architects to decide the taxonomy and information architecture for Azure resources and resource groups. They should identify the categories by which resources will be classified based on the query requirements. However, they must also identify whether tags should be attached to individual resources or to resource groups.
If all resources within a resource group need the same tag, then it is better to tag the resource group, rather than tagging each resource. It is important to take the queries on tags into consideration before finalizing whether tags should be applied at the resource level or the resource group level. If the queries relate to individual resource types across a subscription and across resource groups, then assigning tags to individual resources makes more sense. However, if identifying resource groups is enough for your queries to be effective, then tags should be applied only to resource groups.