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	$25.56	9.126%	$27.8926	$27.89	$27.90	$27.90
Purchase #2	$65.42	10.281%	$72.1458	$72.15	$72.15	$72.10
Purchase #3	$41.67	12.441%	$46.8542	$46.85	$46.85	$46.90
Purchase #4	$147.77	12.696%	$166.5309	$166.53	$166.55	$166.50
Purchase #5	$48.27	13.595%	$54.8323	$54.83	$54.85	$54.80
Purchase #6	$19.26	12.725%	$21.7108	$21.71	$21.70	$21.70
Purchase #7	$222.53	12.447%	$250.2283	$250.23	$250.25	$250.20
7 purchase totals =	$570.48		$640.1949	$640.19	$640.25	$640.10