Also on this I think the ticket total should show the total for money or paid credit tendered.
For tax legalities the discount could be applied before or after tax (include tax checkbox on calculation type) depending on your countries laws.
Using round in default setup even if you round 90% of ticket total the tax is calculated from total before discount, which I had to change for my country.
Maybe i am looking at this completely wrong but i cant see the difference between a % discount or round with free vouchers or earned store credit.
And if modifying ticket total is not the way to do it then should round or discounts not be set up as tenders?