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	$108.68	11.245%	$120.9011	$120.90	$120.90	$120.90
Purchase #2	$33.38	10.667%	$36.9406	$36.94	$36.95	$36.90
Purchase #3	$229.95	13.425%	$260.8208	$260.82	$260.80	$260.80
Purchase #4	$114.48	12.221%	$128.4706	$128.47	$128.45	$128.50
Purchase #5	$106.61	6.140%	$113.1559	$113.16	$113.15	$113.20
Purchase #6	$79.60	12.116%	$89.2443	$89.24	$89.25	$89.20
Purchase #7	$80.60	12.246%	$90.4703	$90.47	$90.45	$90.50
7 purchase totals =	$753.30		$840.0036	$840.00	$839.95	$840.00