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	$11.77	9.610%	$12.9011	$12.90	$12.90	$12.90
Purchase #2	$56.38	3.638%	$58.4311	$58.43	$58.45	$58.40
Purchase #3	$102.33	7.329%	$109.8298	$109.83	$109.85	$109.80
Purchase #4	$126.34	11.108%	$140.3738	$140.37	$140.35	$140.40
Purchase #5	$245.69	8.401%	$266.3304	$266.33	$266.35	$266.30
Purchase #6	$19.59	3.183%	$20.2135	$20.21	$20.20	$20.20
Purchase #7	$49.90	5.671%	$52.7298	$52.73	$52.75	$52.70
7 purchase totals =	$612.00		$660.8095	$660.80	$660.85	$660.70