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	$19.34	8.661%	$21.015	$21.02	$21.00	$21.00
Purchase #2	$168.69	4.487%	$176.2591	$176.26	$176.25	$176.30
Purchase #3	$103.52	9.854%	$113.7209	$113.72	$113.70	$113.70
Purchase #4	$243.47	3.479%	$251.9403	$251.94	$251.95	$251.90
Purchase #5	$73.78	6.703%	$78.7255	$78.73	$78.75	$78.70
Purchase #6	$40.82	4.446%	$42.6349	$42.63	$42.65	$42.60
Purchase #7	$86.64	10.592%	$95.8169	$95.82	$95.80	$95.80
7 purchase totals =	$736.26		$780.1126	$780.12	$780.10	$780.00