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	$25.57	6.770%	$27.3011	$27.30	$27.30	$27.30
Purchase #2	$28.86	13.202%	$32.6701	$32.67	$32.65	$32.70
Purchase #3	$106.98	4.532%	$111.8283	$111.83	$111.85	$111.80
Purchase #4	$33.75	5.297%	$35.5377	$35.54	$35.55	$35.50
Purchase #5	$163.25	5.152%	$171.6606	$171.66	$171.65	$171.70
Purchase #6	$171.65	9.611%	$188.1473	$188.15	$188.15	$188.10
Purchase #7	$85.99	5.279%	$90.5294	$90.53	$90.55	$90.50
7 purchase totals =	$616.05		$657.6745	$657.68	$657.70	$657.60