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	$18.27	12.184%	$20.496	$20.50	$20.50	$20.50
Purchase #2	$165.61	11.729%	$185.0344	$185.03	$185.05	$185.00
Purchase #3	$178.88	5.598%	$188.8937	$188.89	$188.90	$188.90
Purchase #4	$243.96	12.452%	$274.3379	$274.34	$274.35	$274.30
Purchase #5	$90.68	8.583%	$98.4631	$98.46	$98.45	$98.50
Purchase #6	$60.68	10.475%	$67.0362	$67.04	$67.05	$67.00
Purchase #7	$14.48	5.597%	$15.2904	$15.29	$15.30	$15.30
7 purchase totals =	$772.56		$849.5517	$849.55	$849.60	$849.50