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	$19.72	6.564%	$21.0144	$21.01	$21.00	$21.00
Purchase #2	$211.46	10.475%	$233.6104	$233.61	$233.60	$233.60
Purchase #3	$131.50	8.432%	$142.5881	$142.59	$142.60	$142.60
Purchase #4	$219.49	11.571%	$244.8872	$244.89	$244.90	$244.90
Purchase #5	$180.45	10.699%	$199.7563	$199.76	$199.75	$199.80
Purchase #6	$96.46	3.405%	$99.7445	$99.74	$99.75	$99.70
Purchase #7	$175.42	12.500%	$197.3475	$197.35	$197.35	$197.30
7 purchase totals =	$1,034.50		$1,138.9484	$1,138.95	$1,138.95	$1,138.90