From a business perspective, custom tags will add to your production cost but it doesn't actually add any value to the consumer. So you want to be careful not to force the retail price too high. At the same time, you don't want to cut into your profit margins either. As a startup brand, your main focus should be on generating cash flow. Make shirts and sell shirts. Rinse and repeat.
From a branding standpoint, I understand where you're coming from. You want your customers to feel like they are getting a quality product. If you have the budget to launch a legit marketing campaign from the beginning, then by all means, pay for custom tags. But if you are on a modest budget, you will need to start small and build from there. True brand building is going to be getting your product out there and creating a demand for more.
You can transition into custom blanks, tags, packaging, etc, when the time is right. But for now, get your shirts out there and start selling.