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	$7.82	4.418%	$8.1655	$8.17	$8.15	$8.20
Purchase #2	$247.80	6.669%	$264.3258	$264.33	$264.35	$264.30
Purchase #3	$78.36	13.807%	$89.1792	$89.18	$89.20	$89.20
Purchase #4	$84.51	8.759%	$91.9122	$91.91	$91.90	$91.90
Purchase #5	$244.44	6.833%	$261.1426	$261.14	$261.15	$261.10
Purchase #6	$173.50	10.429%	$191.5943	$191.59	$191.60	$191.60
Purchase #7	$30.35	10.778%	$33.6211	$33.62	$33.60	$33.60
7 purchase totals =	$866.78		$939.9407	$939.94	$939.95	$939.90