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	$18.63	12.385%	$20.9373	$20.94	$20.95	$20.90
Purchase #2	$93.92	7.339%	$100.8128	$100.81	$100.80	$100.80
Purchase #3	$94.91	4.509%	$99.1895	$99.19	$99.20	$99.20
Purchase #4	$228.69	13.478%	$259.5128	$259.51	$259.50	$259.50
Purchase #5	$125.39	8.255%	$135.7409	$135.74	$135.75	$135.70
Purchase #6	$179.40	9.588%	$196.6009	$196.60	$196.60	$196.60
Purchase #7	$37.83	6.326%	$40.2231	$40.22	$40.20	$40.20
7 purchase totals =	$778.77		$853.0173	$853.01	$853.00	$852.90