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	$26.36	5.788%	$27.8857	$27.89	$27.90	$27.90
Purchase #2	$142.98	10.837%	$158.4747	$158.47	$158.45	$158.50
Purchase #3	$238.95	12.389%	$268.5535	$268.55	$268.55	$268.60
Purchase #4	$156.50	8.829%	$170.3174	$170.32	$170.30	$170.30
Purchase #5	$206.78	13.117%	$233.9033	$233.90	$233.90	$233.90
Purchase #6	$38.92	10.162%	$42.8751	$42.88	$42.90	$42.90
Purchase #7	$64.74	3.225%	$66.8279	$66.83	$66.85	$66.80
7 purchase totals =	$875.23		$968.8376	$968.84	$968.85	$968.90