Multi-Tenant Design with Single DB & Single Schema approach using Sql Server
I would go with including Tenant_ID in all tables. Yes, this is denormalized, but with good reason.

In cases like this, it can be critical to ensure that one tenant cannot see another's data. If every query includes the Tenant_ID, that helps ensure this.

Your note on future partitions is another reason (though not critical at this point).

I can think of one other. If one of your tenants required some sort of direct SQL access to the database, you could create views for them for all tables, with WHERE Tenant_ID = 12 (or whatever the appropriate ID is, of course). By granting access to those views, and denying access to the actual tables, you would be able to provide the same security as you do with your application.