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	$7.67	10.842%	$8.5016	$8.50	$8.50	$8.50
Purchase #2	$234.35	5.760%	$247.8486	$247.85	$247.85	$247.80
Purchase #3	$80.38	11.660%	$89.7523	$89.75	$89.75	$89.80
Purchase #4	$203.50	9.557%	$222.9485	$222.95	$222.95	$222.90
Purchase #5	$145.74	9.700%	$159.8768	$159.88	$159.90	$159.90
Purchase #6	$6.82	5.706%	$7.2091	$7.21	$7.20	$7.20
Purchase #7	$89.26	12.587%	$100.4952	$100.50	$100.50	$100.50
7 purchase totals =	$767.72		$836.6321	$836.64	$836.65	$836.60