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	$38.56	9.355%	$42.1673	$42.17	$42.15	$42.20
Purchase #2	$49.67	12.382%	$55.8201	$55.82	$55.80	$55.80
Purchase #3	$66.97	8.399%	$72.5948	$72.59	$72.60	$72.60
Purchase #4	$157.81	4.837%	$165.4433	$165.44	$165.45	$165.40
Purchase #5	$18.40	7.778%	$19.8312	$19.83	$19.85	$19.80
Purchase #6	$79.30	13.164%	$89.7391	$89.74	$89.75	$89.70
Purchase #7	$56.90	13.855%	$64.7835	$64.78	$64.80	$64.80
7 purchase totals =	$467.61		$510.3793	$510.37	$510.40	$510.30