You can upstream if you choose to the benefit it gives is the high pressure impact of he detergents. That is a good option for fleet washing when you only have cold water to work with. For applying a spray wax that really is not necessary to apply at pressure and you should still give a light rinse after application so down streaming would probably be a better choice in that case.
In most cases, with the proper mix, you can clean as well as upstreaming with a downstream injector and protect your pump and equipment.