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	$5.83	13.281%	$6.6043	$6.60	$6.60	$6.60
Purchase #2	$100.81	10.377%	$111.2711	$111.27	$111.25	$111.30
Purchase #3	$175.29	8.410%	$190.0319	$190.03	$190.05	$190.00
Purchase #4	$102.96	10.317%	$113.5824	$113.58	$113.60	$113.60
Purchase #5	$233.41	7.596%	$251.1398	$251.14	$251.15	$251.10
Purchase #6	$131.55	6.684%	$140.3428	$140.34	$140.35	$140.30
Purchase #7	$227.61	5.585%	$240.322	$240.32	$240.30	$240.30
7 purchase totals =	$977.46		$1,053.2943	$1,053.28	$1,053.30	$1,053.20