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	$79.69	11.676%	$88.9946	$88.99	$89.00	$89.00
Purchase #2	$61.86	13.849%	$70.427	$70.43	$70.45	$70.40
Purchase #3	$62.25	4.738%	$65.1994	$65.20	$65.20	$65.20
Purchase #4	$142.97	10.745%	$158.3321	$158.33	$158.35	$158.30
Purchase #5	$56.62	5.493%	$59.7301	$59.73	$59.75	$59.70
Purchase #6	$44.57	12.449%	$50.1185	$50.12	$50.10	$50.10
Purchase #7	$43.85	8.349%	$47.511	$47.51	$47.50	$47.50
7 purchase totals =	$491.81		$540.3127	$540.31	$540.35	$540.20