There are basically 2 types of hang tags:
1) Retail required type
2) Branding enhancement type
Unless you are selling to major retailers, you do not need to worry about the first... these hang tags require EDI compliant info and specific SKU #s, etc.
For most new brands, you should look to use your hang tag as a way to increase the perceived value of your product. The hang tag is a great way to include a story about your brand, the garment production process, and pump up your web site or social media community.
Some brands use hang tags as a way to enhance the packaging of their product or include important info (i.e. limited edition status) on the specific product. If you go through the process of designing and producing hang tags you should look into using unique materials that complement the vibe of your shirts or brand.
We once produced hang tags for a vintage travel collection of t-shirts and designed each hang tag to look and feel like a vintage luggage tag with the airport code for each destination the artwork originated from; it was a hit with retailers.
We have also produced very elaborate hang tags with a brand's story on it and later found out the retailers either took them off the shirts when placing on the shelves (because it was too difficult to refold with the hang tags each time) or they placed an ugly sticker with the price and bar code over the content on the hang tag. In this case, these hang tags were a waste of $$ for the customer.
Nevertheless, if you want to step up the perception of your brand you should consider stepping up the presentation and quality of your product.
AM