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	$18.86	13.738%	$21.451	$21.45	$21.45	$21.50
Purchase #2	$123.86	13.741%	$140.8796	$140.88	$140.90	$140.90
Purchase #3	$96.65	12.791%	$109.0125	$109.01	$109.00	$109.00
Purchase #4	$71.64	12.179%	$80.365	$80.37	$80.35	$80.40
Purchase #5	$90.62	13.697%	$103.0322	$103.03	$103.05	$103.00
Purchase #6	$109.59	6.769%	$117.0081	$117.01	$117.00	$117.00
Purchase #7	$69.25	7.255%	$74.2741	$74.27	$74.25	$74.30
7 purchase totals =	$580.47		$646.0225	$646.02	$646.00	$646.10