This is configurable in the point of sale system. In my experience at local places it seems relatively evenly split between pretax and posttax. Not clear if there's a consistent default across all the various POS systems.

For example, Square's documentation: https://squareup.com/help/us/en/article/5069-accept-tips-wit...

>You will need to select if you would like your tips to be added before or after your taxes have been applied to the sale.