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	$18.59	10.130%	$20.4732	$20.47	$20.45	$20.50
Purchase #2	$123.61	7.815%	$133.2701	$133.27	$133.25	$133.30
Purchase #3	$48.69	12.563%	$54.8069	$54.81	$54.80	$54.80
Purchase #4	$70.82	13.808%	$80.5988	$80.60	$80.60	$80.60
Purchase #5	$223.42	8.802%	$243.0854	$243.09	$243.10	$243.10
Purchase #6	$62.44	11.497%	$69.6187	$69.62	$69.60	$69.60
Purchase #7	$57.35	11.770%	$64.1001	$64.10	$64.10	$64.10
7 purchase totals =	$604.92		$665.9532	$665.96	$665.90	$666.00