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	$84.63	6.720%	$90.3171	$90.32	$90.30	$90.30
Purchase #2	$93.94	10.320%	$103.6346	$103.63	$103.65	$103.60
Purchase #3	$171.35	11.754%	$191.4905	$191.49	$191.50	$191.50
Purchase #4	$228.26	11.655%	$254.8637	$254.86	$254.85	$254.90
Purchase #5	$48.76	11.209%	$54.2255	$54.23	$54.25	$54.20
Purchase #6	$37.79	4.490%	$39.4868	$39.49	$39.50	$39.50
Purchase #7	$55.64	6.834%	$59.4424	$59.44	$59.45	$59.40
7 purchase totals =	$720.37		$793.4606	$793.46	$793.50	$793.40