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	$5.96	6.390%	$6.3408	$6.34	$6.35	$6.30
Purchase #2	$39.66	7.517%	$42.6412	$42.64	$42.65	$42.60
Purchase #3	$106.90	12.463%	$120.2229	$120.22	$120.20	$120.20
Purchase #4	$231.58	9.697%	$254.0363	$254.04	$254.05	$254.00
Purchase #5	$147.45	7.578%	$158.6238	$158.62	$158.60	$158.60
Purchase #6	$72.64	4.439%	$75.8645	$75.86	$75.85	$75.90
Purchase #7	$69.38	13.708%	$78.8906	$78.89	$78.90	$78.90
7 purchase totals =	$673.57		$736.6201	$736.61	$736.60	$736.50