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	$19.83	7.813%	$21.3793	$21.38	$21.40	$21.40
Purchase #2	$156.26	10.133%	$172.0938	$172.09	$172.10	$172.10
Purchase #3	$58.70	9.570%	$64.3176	$64.32	$64.30	$64.30
Purchase #4	$239.80	6.665%	$255.7827	$255.78	$255.80	$255.80
Purchase #5	$38.46	7.797%	$41.4587	$41.46	$41.45	$41.50
Purchase #6	$139.45	12.820%	$157.3275	$157.33	$157.35	$157.30
Purchase #7	$170.27	12.214%	$191.0668	$191.07	$191.05	$191.10
7 purchase totals =	$822.77		$903.4264	$903.43	$903.45	$903.50