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	$37.75	5.480%	$39.8187	$39.82	$39.80	$39.80
Purchase #2	$142.62	6.452%	$151.8218	$151.82	$151.80	$151.80
Purchase #3	$51.89	5.683%	$54.8389	$54.84	$54.85	$54.80
Purchase #4	$75.51	11.625%	$84.288	$84.29	$84.30	$84.30
Purchase #5	$149.81	11.811%	$167.5041	$167.50	$167.50	$167.50
Purchase #6	$55.58	9.783%	$61.0174	$61.02	$61.00	$61.00
Purchase #7	$249.94	12.284%	$280.6426	$280.64	$280.65	$280.60
7 purchase totals =	$763.10		$839.9315	$839.93	$839.90	$839.80