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	13.748%	$21.4756	$21.48	$21.50	$21.50
Purchase #2	$68.27	6.662%	$72.8181	$72.82	$72.80	$72.80
Purchase #3	$194.91	10.218%	$214.8259	$214.83	$214.85	$214.80
Purchase #4	$217.95	6.793%	$232.7553	$232.76	$232.75	$232.80
Purchase #5	$63.52	4.754%	$66.5397	$66.54	$66.55	$66.50
Purchase #6	$131.64	10.322%	$145.2279	$145.23	$145.25	$145.20
Purchase #7	$81.69	11.524%	$91.104	$91.10	$91.10	$91.10
7 purchase totals =	$776.86		$844.7465	$844.76	$844.80	$844.70