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	$38.49	10.343%	$42.471	$42.47	$42.45	$42.50
Purchase #2	$142.65	10.295%	$157.3358	$157.34	$157.35	$157.30
Purchase #3	$38.96	8.496%	$42.27	$42.27	$42.25	$42.30
Purchase #4	$167.48	13.574%	$190.2137	$190.21	$190.20	$190.20
Purchase #5	$135.34	8.436%	$146.7573	$146.76	$146.75	$146.80
Purchase #6	$90.81	9.460%	$99.4006	$99.40	$99.40	$99.40
Purchase #7	$202.37	4.803%	$212.0898	$212.09	$212.10	$212.10
7 purchase totals =	$816.10		$890.5382	$890.54	$890.50	$890.60