It depends on the local legislation of the country where you are operating. For example in most countries in Europe, the principle is as follows:
- You don't have to if you are operating under a "VAT not applicable" scheme (in France for example: individuals and most nonprofits).
- You do have to if your rewards are considered as purchases (see Is my donation considered an investment?) and if you are a business, or an organisation with commercial activities. In this case, the rate to apply depends on the sector of activity of your project.