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	$16.67	11.120%	$18.5237	$18.52	$18.50	$18.50
Purchase #2	$105.91	3.658%	$109.7842	$109.78	$109.80	$109.80
Purchase #3	$249.84	11.829%	$279.3936	$279.39	$279.40	$279.40
Purchase #4	$245.65	3.710%	$254.7636	$254.76	$254.75	$254.80
Purchase #5	$31.77	9.691%	$34.8488	$34.85	$34.85	$34.80
Purchase #6	$39.63	3.504%	$41.0186	$41.02	$41.00	$41.00
Purchase #7	$140.38	10.826%	$155.5775	$155.58	$155.60	$155.60
7 purchase totals =	$829.85		$893.9100	$893.90	$893.90	$893.90