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	$16.81	12.231%	$18.866	$18.87	$18.85	$18.90
Purchase #2	$223.46	13.812%	$254.3243	$254.32	$254.30	$254.30
Purchase #3	$81.26	10.786%	$90.0247	$90.02	$90.00	$90.00
Purchase #4	$234.73	13.763%	$267.0359	$267.04	$267.05	$267.00
Purchase #5	$100.48	8.452%	$108.9726	$108.97	$108.95	$109.00
Purchase #6	$235.73	5.612%	$248.9592	$248.96	$248.95	$249.00
Purchase #7	$27.91	12.676%	$31.4479	$31.45	$31.45	$31.40
7 purchase totals =	$920.38		$1,019.6306	$1,019.63	$1,019.55	$1,019.60