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	$244.35	11.831%	$273.259	$273.26	$273.25	$273.30
Purchase #2	$155.81	3.527%	$161.3054	$161.31	$161.30	$161.30
Purchase #3	$36.67	3.379%	$37.9091	$37.91	$37.90	$37.90
Purchase #4	$86.72	5.628%	$91.6006	$91.60	$91.60	$91.60
Purchase #5	$208.32	11.345%	$231.9539	$231.95	$231.95	$232.00
Purchase #6	$18.26	9.566%	$20.0068	$20.01	$20.00	$20.00
Purchase #7	$22.89	4.385%	$23.8937	$23.89	$23.90	$23.90
7 purchase totals =	$773.02		$839.9285	$839.93	$839.90	$840.00