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	$18.94	12.522%	$21.3117	$21.31	$21.30	$21.30
Purchase #2	$85.76	12.143%	$96.1738	$96.17	$96.15	$96.20
Purchase #3	$106.49	12.159%	$119.4381	$119.44	$119.45	$119.40
Purchase #4	$70.57	13.184%	$79.8739	$79.87	$79.85	$79.90
Purchase #5	$39.31	5.155%	$41.3364	$41.34	$41.35	$41.30
Purchase #6	$83.59	8.148%	$90.4009	$90.40	$90.40	$90.40
Purchase #7	$188.73	10.646%	$208.8222	$208.82	$208.80	$208.80
7 purchase totals =	$593.39		$657.3570	$657.35	$657.30	$657.30