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	$89.93	10.435%	$99.3142	$99.31	$99.30	$99.30
Purchase #2	$135.65	11.574%	$151.3501	$151.35	$151.35	$151.40
Purchase #3	$131.81	5.704%	$139.3284	$139.33	$139.35	$139.30
Purchase #4	$94.47	11.852%	$105.6666	$105.67	$105.65	$105.70
Purchase #5	$138.90	10.717%	$153.7859	$153.79	$153.80	$153.80
Purchase #6	$77.57	8.558%	$84.2084	$84.21	$84.20	$84.20
Purchase #7	$54.50	3.228%	$56.2593	$56.26	$56.25	$56.30
7 purchase totals =	$722.83		$789.9129	$789.92	$789.90	$790.00