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	$18.41	8.683%	$20.0085	$20.01	$20.00	$20.00
Purchase #2	$140.75	12.278%	$158.0313	$158.03	$158.05	$158.00
Purchase #3	$54.61	11.593%	$60.9409	$60.94	$60.95	$60.90
Purchase #4	$184.96	6.366%	$196.7346	$196.73	$196.75	$196.70
Purchase #5	$59.85	12.830%	$67.5288	$67.53	$67.55	$67.50
Purchase #6	$50.43	12.555%	$56.7615	$56.76	$56.75	$56.80
Purchase #7	$131.86	10.604%	$145.8424	$145.84	$145.85	$145.80
7 purchase totals =	$640.87		$705.8480	$705.84	$705.90	$705.70