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	$18.92	11.666%	$21.1272	$21.13	$21.15	$21.10
Purchase #2	$101.55	7.806%	$109.477	$109.48	$109.50	$109.50
Purchase #3	$138.54	8.871%	$150.8299	$150.83	$150.85	$150.80
Purchase #4	$209.61	13.705%	$238.3371	$238.34	$238.35	$238.30
Purchase #5	$69.45	9.571%	$76.0971	$76.10	$76.10	$76.10
Purchase #6	$117.77	13.354%	$133.497	$133.50	$133.50	$133.50
Purchase #7	$206.28	11.705%	$230.4251	$230.43	$230.45	$230.40
7 purchase totals =	$862.12		$959.7904	$959.81	$959.90	$959.70