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	$18.55	13.196%	$20.9979	$21.00	$21.00	$21.00
Purchase #2	$196.71	8.473%	$213.3772	$213.38	$213.40	$213.40
Purchase #3	$116.69	11.638%	$130.2704	$130.27	$130.25	$130.30
Purchase #4	$192.67	3.740%	$199.8759	$199.88	$199.90	$199.90
Purchase #5	$130.83	9.650%	$143.4551	$143.46	$143.45	$143.50
Purchase #6	$74.56	3.193%	$76.9407	$76.94	$76.95	$76.90
Purchase #7	$104.39	13.568%	$118.5536	$118.55	$118.55	$118.60
7 purchase totals =	$834.40		$903.4708	$903.48	$903.50	$903.60