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	$25.66	3.344%	$26.5181	$26.52	$26.50	$26.50
Purchase #2	$197.76	3.150%	$203.9894	$203.99	$204.00	$204.00
Purchase #3	$100.96	10.712%	$111.7748	$111.77	$111.75	$111.80
Purchase #4	$115.98	10.599%	$128.2727	$128.27	$128.25	$128.30
Purchase #5	$73.75	13.220%	$83.4998	$83.50	$83.50	$83.50
Purchase #6	$95.48	4.103%	$99.3975	$99.40	$99.40	$99.40
Purchase #7	$160.63	3.582%	$166.3838	$166.38	$166.40	$166.40
7 purchase totals =	$770.22		$819.8361	$819.83	$819.80	$819.90