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	$25.58	9.457%	$27.9991	$28.00	$28.00	$28.00
Purchase #2	$67.46	4.270%	$70.3405	$70.34	$70.35	$70.30
Purchase #3	$122.65	11.295%	$136.5033	$136.50	$136.50	$136.50
Purchase #4	$35.76	13.347%	$40.5329	$40.53	$40.55	$40.50
Purchase #5	$114.67	11.823%	$128.2274	$128.23	$128.25	$128.20
Purchase #6	$172.93	4.157%	$180.1187	$180.12	$180.10	$180.10
Purchase #7	$6.50	13.732%	$7.3926	$7.39	$7.40	$7.40
7 purchase totals =	$545.55		$591.1145	$591.11	$591.15	$591.00