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	$240.31	12.476%	$270.2911	$270.29	$270.30	$270.30
Purchase #2	$152.74	12.320%	$171.5576	$171.56	$171.55	$171.60
Purchase #3	$86.46	10.196%	$95.2755	$95.28	$95.30	$95.30
Purchase #4	$204.93	5.263%	$215.7155	$215.72	$215.70	$215.70
Purchase #5	$61.25	6.291%	$65.1032	$65.10	$65.10	$65.10
Purchase #6	$215.25	3.402%	$222.5728	$222.57	$222.55	$222.60
Purchase #7	$159.75	13.525%	$181.3562	$181.36	$181.35	$181.40
7 purchase totals =	$1,120.69		$1,221.8719	$1,221.88	$1,221.85	$1,222.00