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	$249.72	12.370%	$280.6104	$280.61	$280.60	$280.60
Purchase #2	$232.47	10.241%	$256.2773	$256.28	$256.30	$256.30
Purchase #3	$50.90	3.619%	$52.7421	$52.74	$52.75	$52.70
Purchase #4	$29.81	11.336%	$33.1893	$33.19	$33.20	$33.20
Purchase #5	$156.80	6.369%	$166.7866	$166.79	$166.80	$166.80
Purchase #6	$31.39	7.831%	$33.8482	$33.85	$33.85	$33.80
Purchase #7	$132.29	11.454%	$147.4425	$147.44	$147.45	$147.40
7 purchase totals =	$883.38		$970.8964	$970.90	$970.95	$970.80