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	$20.70	8.664%	$22.4934	$22.49	$22.50	$22.50
Purchase #2	$138.40	11.794%	$154.7229	$154.72	$154.70	$154.70
Purchase #3	$157.31	7.342%	$168.8597	$168.86	$168.85	$168.90
Purchase #4	$129.73	10.696%	$143.6059	$143.61	$143.60	$143.60
Purchase #5	$42.91	10.205%	$47.289	$47.29	$47.30	$47.30
Purchase #6	$110.86	6.345%	$117.8941	$117.89	$117.90	$117.90
Purchase #7	$240.70	12.590%	$271.0041	$271.00	$271.00	$271.00
7 purchase totals =	$840.61		$925.8691	$925.86	$925.85	$925.90