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	$124.25	12.130%	$139.3215	$139.32	$139.30	$139.30
Purchase #2	$95.63	10.514%	$105.6845	$105.68	$105.70	$105.70
Purchase #3	$249.63	11.736%	$278.9266	$278.93	$278.95	$278.90
Purchase #4	$54.45	6.824%	$58.1657	$58.17	$58.15	$58.20
Purchase #5	$82.98	8.627%	$90.1387	$90.14	$90.15	$90.10
Purchase #6	$217.26	12.153%	$243.6636	$243.66	$243.65	$243.70
Purchase #7	$245.67	12.299%	$275.885	$275.89	$275.90	$275.90
7 purchase totals =	$1,069.87		$1,191.7856	$1,191.79	$1,191.80	$1,191.80