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	$7.51	4.232%	$7.8278	$7.83	$7.85	$7.80
Purchase #2	$140.70	12.346%	$158.0708	$158.07	$158.05	$158.10
Purchase #3	$29.30	9.615%	$32.1172	$32.12	$32.10	$32.10
Purchase #4	$185.52	12.830%	$209.3222	$209.32	$209.30	$209.30
Purchase #5	$34.69	10.219%	$38.235	$38.24	$38.25	$38.20
Purchase #6	$71.47	12.621%	$80.4902	$80.49	$80.50	$80.50
Purchase #7	$134.55	7.172%	$144.1999	$144.20	$144.20	$144.20
7 purchase totals =	$603.74		$670.2631	$670.27	$670.25	$670.20