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	$36.33	8.431%	$39.393	$39.39	$39.40	$39.40
Purchase #2	$29.61	9.487%	$32.4191	$32.42	$32.40	$32.40
Purchase #3	$80.28	12.748%	$90.5141	$90.51	$90.50	$90.50
Purchase #4	$6.41	13.405%	$7.2693	$7.27	$7.25	$7.30
Purchase #5	$23.44	10.124%	$25.8131	$25.81	$25.80	$25.80
Purchase #6	$179.25	8.856%	$195.1244	$195.12	$195.10	$195.10
Purchase #7	$247.59	13.235%	$280.3585	$280.36	$280.35	$280.40
7 purchase totals =	$602.91		$670.8915	$670.88	$670.80	$670.90