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	$11.90	5.113%	$12.5084	$12.51	$12.50	$12.50
Purchase #2	$199.74	10.383%	$220.479	$220.48	$220.50	$220.50
Purchase #3	$16.86	9.247%	$18.419	$18.42	$18.40	$18.40
Purchase #4	$52.62	4.301%	$54.8832	$54.88	$54.90	$54.90
Purchase #5	$70.25	11.363%	$78.2325	$78.23	$78.25	$78.20
Purchase #6	$161.66	8.166%	$174.8612	$174.86	$174.85	$174.90
Purchase #7	$83.81	10.135%	$92.3041	$92.30	$92.30	$92.30
7 purchase totals =	$596.84		$651.6874	$651.68	$651.70	$651.70