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	$27.89	12.577%	$31.3977	$31.40	$31.40	$31.40
Purchase #2	$159.89	11.658%	$178.53	$178.53	$178.55	$178.50
Purchase #3	$137.81	10.418%	$152.167	$152.17	$152.15	$152.20
Purchase #4	$99.71	3.602%	$103.3016	$103.30	$103.30	$103.30
Purchase #5	$156.42	12.504%	$175.9788	$175.98	$176.00	$176.00
Purchase #6	$61.70	11.542%	$68.8214	$68.82	$68.80	$68.80
Purchase #7	$43.42	13.533%	$49.296	$49.30	$49.30	$49.30
7 purchase totals =	$686.84		$759.4925	$759.50	$759.50	$759.50