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	$249.52	12.201%	$279.9639	$279.96	$279.95	$280.00
Purchase #2	$28.59	7.431%	$30.7145	$30.71	$30.70	$30.70
Purchase #3	$113.33	9.688%	$124.3094	$124.31	$124.30	$124.30
Purchase #4	$182.85	12.628%	$205.9403	$205.94	$205.95	$205.90
Purchase #5	$216.25	8.358%	$234.3242	$234.32	$234.30	$234.30
Purchase #6	$187.83	4.750%	$196.7519	$196.75	$196.75	$196.80
Purchase #7	$51.60	8.358%	$55.9127	$55.91	$55.90	$55.90
7 purchase totals =	$1,029.97		$1,127.9169	$1,127.90	$1,127.85	$1,127.90