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	$112.96	6.673%	$120.4978	$120.50	$120.50	$120.50
Purchase #2	$94.36	13.285%	$106.8957	$106.90	$106.90	$106.90
Purchase #3	$160.76	11.165%	$178.7089	$178.71	$178.70	$178.70
Purchase #4	$120.69	8.711%	$131.2033	$131.20	$131.20	$131.20
Purchase #5	$46.42	3.705%	$48.1399	$48.14	$48.15	$48.10
Purchase #6	$37.48	12.134%	$42.0278	$42.03	$42.05	$42.00
Purchase #7	$37.86	9.791%	$41.5669	$41.57	$41.55	$41.60
7 purchase totals =	$610.53		$669.0403	$669.05	$669.05	$669.00