Let's do some math ..  Rounding examples of 7 random hypothetical purchases:
Case	Tag   Price	Sales   Tax %	Raw Total   Incl. Sales Tax	Rounded   to Penny	Rounded   to Nickel	Rounded   to Dime
Purchase #1	$18.83	11.642%	$21.0222	$21.02	$21.00	$21.00
Purchase #2	$79.96	4.666%	$83.6909	$83.69	$83.70	$83.70
Purchase #3	$52.98	11.702%	$59.1797	$59.18	$59.20	$59.20
Purchase #4	$23.57	6.343%	$25.065	$25.07	$25.05	$25.10
Purchase #5	$155.56	13.787%	$177.0071	$177.01	$177.00	$177.00
Purchase #6	$49.57	10.839%	$54.9429	$54.94	$54.95	$54.90
Purchase #7	$53.66	11.612%	$59.891	$59.89	$59.90	$59.90
7 purchase totals =	$434.13		$480.7988	$480.80	$480.80	$480.80