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	$34.70	13.748%	$39.4706	$39.47	$39.45	$39.50
Purchase #2	$163.55	13.241%	$185.2057	$185.21	$185.20	$185.20
Purchase #3	$55.67	11.306%	$61.9641	$61.96	$61.95	$62.00
Purchase #4	$76.84	7.288%	$82.4401	$82.44	$82.45	$82.40
Purchase #5	$155.90	4.133%	$162.3433	$162.34	$162.35	$162.30
Purchase #6	$71.37	12.317%	$80.1606	$80.16	$80.15	$80.20
Purchase #7	$238.58	8.468%	$258.783	$258.78	$258.80	$258.80
7 purchase totals =	$796.61		$870.3674	$870.36	$870.35	$870.40