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	$38.25	6.749%	$40.8315	$40.83	$40.85	$40.80
Purchase #2	$57.39	5.619%	$60.6147	$60.61	$60.60	$60.60
Purchase #3	$81.70	9.635%	$89.5718	$89.57	$89.55	$89.60
Purchase #4	$221.39	8.506%	$240.2214	$240.22	$240.20	$240.20
Purchase #5	$42.70	11.733%	$47.71	$47.71	$47.70	$47.70
Purchase #6	$131.82	6.207%	$140.0021	$140.00	$140.00	$140.00
Purchase #7	$156.89	8.342%	$169.9778	$169.98	$170.00	$170.00
7 purchase totals =	$730.14		$788.9293	$788.92	$788.90	$788.90