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	$28.52	6.850%	$30.4736	$30.47	$30.45	$30.50
Purchase #2	$131.25	5.791%	$138.8507	$138.85	$138.85	$138.90
Purchase #3	$53.48	10.629%	$59.1644	$59.16	$59.15	$59.20
Purchase #4	$107.27	3.253%	$110.7595	$110.76	$110.75	$110.80
Purchase #5	$85.76	3.172%	$88.4803	$88.48	$88.50	$88.50
Purchase #6	$78.54	5.552%	$82.9005	$82.90	$82.90	$82.90
Purchase #7	$72.77	9.379%	$79.5951	$79.60	$79.60	$79.60
7 purchase totals =	$557.59		$590.2241	$590.22	$590.20	$590.40