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	$19.54	7.871%	$21.078	$21.08	$21.10	$21.10
Purchase #2	$115.89	13.139%	$131.1168	$131.12	$131.10	$131.10
Purchase #3	$148.44	3.384%	$153.4632	$153.46	$153.45	$153.50
Purchase #4	$109.84	9.586%	$120.3693	$120.37	$120.35	$120.40
Purchase #5	$139.73	11.174%	$155.3434	$155.34	$155.35	$155.30
Purchase #6	$109.62	6.246%	$116.4669	$116.47	$116.45	$116.50
Purchase #7	$169.50	13.314%	$192.0672	$192.07	$192.05	$192.10
7 purchase totals =	$812.56		$889.9048	$889.91	$889.85	$890.00