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	$38.98	9.629%	$42.7334	$42.73	$42.75	$42.70
Purchase #2	$23.76	7.858%	$25.6271	$25.63	$25.65	$25.60
Purchase #3	$33.44	13.318%	$37.8935	$37.89	$37.90	$37.90
Purchase #4	$204.51	13.425%	$231.9655	$231.97	$231.95	$232.00
Purchase #5	$189.67	3.397%	$196.1131	$196.11	$196.10	$196.10
Purchase #6	$7.54	3.651%	$7.8153	$7.82	$7.80	$7.80
Purchase #7	$88.67	10.382%	$97.8757	$97.88	$97.90	$97.90
7 purchase totals =	$586.57		$640.0236	$640.03	$640.05	$640.00