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	$19.56	11.856%	$21.879	$21.88	$21.90	$21.90
Purchase #2	$173.60	13.396%	$196.8555	$196.86	$196.85	$196.90
Purchase #3	$244.93	8.254%	$265.1465	$265.15	$265.15	$265.10
Purchase #4	$201.28	4.653%	$210.6456	$210.65	$210.65	$210.60
Purchase #5	$139.55	3.672%	$144.6743	$144.67	$144.65	$144.70
Purchase #6	$186.61	10.274%	$205.7823	$205.78	$205.80	$205.80
Purchase #7	$157.94	6.535%	$168.2614	$168.26	$168.25	$168.30
7 purchase totals =	$1,123.47		$1,213.2446	$1,213.25	$1,213.25	$1,213.30