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	$37.31	7.205%	$39.9982	$40.00	$40.00	$40.00
Purchase #2	$59.36	4.343%	$61.938	$61.94	$61.95	$61.90
Purchase #3	$19.37	11.696%	$21.6355	$21.64	$21.65	$21.60
Purchase #4	$129.89	9.370%	$142.0607	$142.06	$142.05	$142.10
Purchase #5	$30.81	3.658%	$31.937	$31.94	$31.95	$31.90
Purchase #6	$147.83	3.807%	$153.4579	$153.46	$153.45	$153.50
Purchase #7	$52.38	12.581%	$58.9699	$58.97	$58.95	$59.00
7 purchase totals =	$476.95		$509.9972	$510.01	$510.00	$510.00