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	$17.68	6.860%	$18.8928	$18.89	$18.90	$18.90
Purchase #2	$128.82	12.586%	$145.0333	$145.03	$145.05	$145.00
Purchase #3	$138.41	3.442%	$143.1741	$143.17	$143.15	$143.20
Purchase #4	$39.57	6.498%	$42.1413	$42.14	$42.15	$42.10
Purchase #5	$202.94	9.611%	$222.4446	$222.44	$222.45	$222.40
Purchase #6	$206.46	12.652%	$232.5813	$232.58	$232.60	$232.60
Purchase #7	$122.58	7.108%	$131.293	$131.29	$131.30	$131.30
7 purchase totals =	$856.46		$935.5604	$935.54	$935.60	$935.50