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	$6.36	3.718%	$6.5965	$6.60	$6.60	$6.60
Purchase #2	$141.62	4.439%	$147.9065	$147.91	$147.90	$147.90
Purchase #3	$65.75	9.756%	$72.1646	$72.16	$72.15	$72.20
Purchase #4	$181.30	6.787%	$193.6048	$193.60	$193.60	$193.60
Purchase #5	$48.40	7.800%	$52.1752	$52.18	$52.20	$52.20
Purchase #6	$168.78	10.180%	$185.9618	$185.96	$185.95	$186.00
Purchase #7	$99.38	12.275%	$111.5789	$111.58	$111.60	$111.60
7 purchase totals =	$711.59		$769.9883	$769.99	$770.00	$770.10