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	$25.30	7.146%	$27.1079	$27.11	$27.10	$27.10
Purchase #2	$123.61	6.721%	$131.9178	$131.92	$131.90	$131.90
Purchase #3	$117.29	7.867%	$126.5172	$126.52	$126.50	$126.50
Purchase #4	$70.84	13.406%	$80.3368	$80.34	$80.35	$80.30
Purchase #5	$80.89	11.457%	$90.1576	$90.16	$90.15	$90.20
Purchase #6	$167.55	6.815%	$178.9685	$178.97	$178.95	$179.00
Purchase #7	$210.71	5.790%	$222.9101	$222.91	$222.90	$222.90
7 purchase totals =	$796.19		$857.9159	$857.93	$857.85	$857.90