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	$16.29	11.224%	$18.1184	$18.12	$18.10	$18.10
Purchase #2	$112.50	4.806%	$117.9068	$117.91	$117.90	$117.90
Purchase #3	$208.27	6.387%	$221.5722	$221.57	$221.55	$221.60
Purchase #4	$124.92	10.671%	$138.2502	$138.25	$138.25	$138.30
Purchase #5	$45.60	11.599%	$50.8891	$50.89	$50.90	$50.90
Purchase #6	$179.30	4.179%	$186.7929	$186.79	$186.80	$186.80
Purchase #7	$95.52	10.299%	$105.3576	$105.36	$105.35	$105.40
7 purchase totals =	$782.40		$838.8872	$838.89	$838.85	$839.00