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	$141.97	12.267%	$159.3855	$159.39	$159.40	$159.40
Purchase #2	$149.79	5.136%	$157.4832	$157.48	$157.50	$157.50
Purchase #3	$166.46	9.808%	$182.7864	$182.79	$182.80	$182.80
Purchase #4	$50.48	10.368%	$55.7138	$55.71	$55.70	$55.70
Purchase #5	$105.72	13.387%	$119.8727	$119.87	$119.85	$119.90
Purchase #6	$33.71	12.210%	$37.826	$37.83	$37.85	$37.80
Purchase #7	$115.55	10.265%	$127.4112	$127.41	$127.40	$127.40
7 purchase totals =	$763.68		$840.4788	$840.48	$840.50	$840.50