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	$35.89	11.174%	$39.9003	$39.90	$39.90	$39.90
Purchase #2	$208.72	8.847%	$227.1855	$227.19	$227.20	$227.20
Purchase #3	$92.89	8.433%	$100.7234	$100.72	$100.70	$100.70
Purchase #4	$133.71	6.245%	$142.0602	$142.06	$142.05	$142.10
Purchase #5	$88.67	11.138%	$98.5461	$98.55	$98.55	$98.50
Purchase #6	$89.36	11.732%	$99.8437	$99.84	$99.85	$99.80
Purchase #7	$79.71	3.799%	$82.7382	$82.74	$82.75	$82.70
7 purchase totals =	$728.95		$790.9974	$791.00	$791.00	$790.90