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	$36.57	9.144%	$39.914	$39.91	$39.90	$39.90
Purchase #2	$111.75	11.599%	$124.7119	$124.71	$124.70	$124.70
Purchase #3	$250.78	9.850%	$275.4818	$275.48	$275.50	$275.50
Purchase #4	$152.92	6.508%	$162.872	$162.87	$162.85	$162.90
Purchase #5	$99.48	13.305%	$112.7158	$112.72	$112.70	$112.70
Purchase #6	$100.28	12.692%	$113.0075	$113.01	$113.00	$113.00
Purchase #7	$127.28	8.312%	$137.8595	$137.86	$137.85	$137.90
7 purchase totals =	$879.06		$966.5625	$966.56	$966.50	$966.60