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	$17.80	3.253%	$18.379	$18.38	$18.40	$18.40
Purchase #2	$143.48	11.872%	$160.5139	$160.51	$160.50	$160.50
Purchase #3	$155.54	6.211%	$165.2006	$165.20	$165.20	$165.20
Purchase #4	$92.68	11.483%	$103.3224	$103.32	$103.30	$103.30
Purchase #5	$133.63	7.663%	$143.8701	$143.87	$143.85	$143.90
Purchase #6	$145.28	3.657%	$150.5929	$150.59	$150.60	$150.60
Purchase #7	$122.33	11.415%	$136.294	$136.29	$136.30	$136.30
7 purchase totals =	$810.74		$878.1729	$878.16	$878.15	$878.20