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	$26.54	12.688%	$29.9074	$29.91	$29.90	$29.90
Purchase #2	$39.34	7.838%	$42.4235	$42.42	$42.40	$42.40
Purchase #3	$109.84	10.149%	$120.9877	$120.99	$121.00	$121.00
Purchase #4	$87.53	11.827%	$97.8822	$97.88	$97.90	$97.90
Purchase #5	$61.87	11.271%	$68.8434	$68.84	$68.85	$68.80
Purchase #6	$192.47	10.291%	$212.2771	$212.28	$212.30	$212.30
Purchase #7	$83.93	13.858%	$95.561	$95.56	$95.55	$95.60
7 purchase totals =	$601.52		$667.8823	$667.88	$667.90	$667.90