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	$18.98	13.266%	$21.4979	$21.50	$21.50	$21.50
Purchase #2	$241.34	11.851%	$269.9412	$269.94	$269.95	$269.90
Purchase #3	$142.70	13.308%	$161.6905	$161.69	$161.70	$161.70
Purchase #4	$48.89	4.146%	$50.917	$50.92	$50.90	$50.90
Purchase #5	$241.72	11.326%	$269.0972	$269.10	$269.10	$269.10
Purchase #6	$246.91	3.860%	$256.4407	$256.44	$256.45	$256.40
Purchase #7	$124.52	10.166%	$137.1787	$137.18	$137.20	$137.20
7 purchase totals =	$1,065.06		$1,166.7632	$1,166.77	$1,166.80	$1,166.70