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	$18.88	11.586%	$21.0674	$21.07	$21.05	$21.10
Purchase #2	$148.56	13.214%	$168.1907	$168.19	$168.20	$168.20
Purchase #3	$37.71	6.391%	$40.12	$40.12	$40.10	$40.10
Purchase #4	$54.68	11.817%	$61.1415	$61.14	$61.15	$61.10
Purchase #5	$90.57	8.580%	$98.3409	$98.34	$98.35	$98.30
Purchase #6	$30.51	5.320%	$32.1331	$32.13	$32.15	$32.10
Purchase #7	$173.48	13.662%	$197.1808	$197.18	$197.20	$197.20
7 purchase totals =	$554.39		$618.1744	$618.17	$618.20	$618.10