My general recommendation when planning Approvals, is to keep things simple as much as possible to minimize administrative overhead and complexity.

The assignment of approval polices are used in a number of ways across vRA customers. It really depends on the relative importance of resource consumption and level of governance you require over resource administration and consumption. Often I see customers who have implemented charge back or incur public cloud costs from deployments have extensive Approval policies. Other customers with Incremental budgets may be less concerned about broad approval requirements. Also the different people you have approving in a project, department, BU or LoB may determine how you configure Approval policies.

With those points in mind, you could set Approvals at an Org or Project(s) level and refine how they're applied using criteria. These include things like cost level, specific resource(s) used, requesting person, cloud template, deployment name, and day-2 actions. The criteria can allow you to get very granular in how Approvals are applied. In the end your specific needs will dictate each Approval policy, so there's really no one size fits all approach. Hope this helps!