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	$36.87	8.117%	$39.8627	$39.86	$39.85	$39.90
Purchase #2	$72.72	10.718%	$80.5141	$80.51	$80.50	$80.50
Purchase #3	$124.61	9.624%	$136.6025	$136.60	$136.60	$136.60
Purchase #4	$21.33	9.536%	$23.364	$23.36	$23.35	$23.40
Purchase #5	$141.84	3.726%	$147.125	$147.13	$147.15	$147.10
Purchase #6	$170.78	12.384%	$191.9294	$191.93	$191.95	$191.90
Purchase #7	$53.59	10.405%	$59.166	$59.17	$59.15	$59.20
7 purchase totals =	$621.74		$678.5637	$678.56	$678.55	$678.60