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	$36.57	6.649%	$39.0015	$39.00	$39.00	$39.00
Purchase #2	$84.37	6.262%	$89.6532	$89.65	$89.65	$89.70
Purchase #3	$42.83	5.387%	$45.1373	$45.14	$45.15	$45.10
Purchase #4	$11.97	9.589%	$13.1178	$13.12	$13.10	$13.10
Purchase #5	$84.50	5.386%	$89.0512	$89.05	$89.05	$89.10
Purchase #6	$97.36	4.233%	$101.4812	$101.48	$101.50	$101.50
Purchase #7	$247.40	5.781%	$261.7022	$261.70	$261.70	$261.70
7 purchase totals =	$605.00		$639.1444	$639.14	$639.15	$639.20