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	$38.46	9.244%	$42.0152	$42.02	$42.00	$42.00
Purchase #2	$85.96	4.601%	$89.915	$89.92	$89.90	$89.90
Purchase #3	$193.53	5.131%	$203.46	$203.46	$203.45	$203.50
Purchase #4	$128.43	10.326%	$141.6917	$141.69	$141.70	$141.70
Purchase #5	$225.29	12.249%	$252.8858	$252.89	$252.90	$252.90
Purchase #6	$60.90	11.230%	$67.7391	$67.74	$67.75	$67.70
Purchase #7	$35.33	12.242%	$39.6551	$39.66	$39.65	$39.70
7 purchase totals =	$767.90		$837.3619	$837.38	$837.35	$837.40