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	$24.96	12.302%	$28.0306	$28.03	$28.05	$28.00
Purchase #2	$114.91	5.120%	$120.7934	$120.79	$120.80	$120.80
Purchase #3	$109.37	8.333%	$118.4838	$118.48	$118.50	$118.50
Purchase #4	$173.45	3.563%	$179.63	$179.63	$179.65	$179.60
Purchase #5	$99.72	6.259%	$105.9615	$105.96	$105.95	$106.00
Purchase #6	$68.91	5.400%	$72.6311	$72.63	$72.65	$72.60
Purchase #7	$42.57	4.440%	$44.4601	$44.46	$44.45	$44.50
7 purchase totals =	$633.89		$669.9905	$669.98	$670.05	$670.00