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	$36.95	8.705%	$40.1665	$40.17	$40.15	$40.20
Purchase #2	$37.67	7.836%	$40.6218	$40.62	$40.60	$40.60
Purchase #3	$117.27	10.291%	$129.3383	$129.34	$129.35	$129.30
Purchase #4	$102.84	12.355%	$115.5459	$115.55	$115.55	$115.50
Purchase #5	$174.57	9.363%	$190.915	$190.92	$190.90	$190.90
Purchase #6	$85.59	4.257%	$89.2336	$89.23	$89.25	$89.20
Purchase #7	$22.77	7.184%	$24.4058	$24.41	$24.40	$24.40
7 purchase totals =	$577.66		$630.2269	$630.24	$630.20	$630.10