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	$80.37	12.484%	$90.4034	$90.40	$90.40	$90.40
Purchase #2	$100.52	3.130%	$103.6663	$103.67	$103.65	$103.70
Purchase #3	$183.44	12.605%	$206.5626	$206.56	$206.55	$206.60
Purchase #4	$142.25	5.857%	$150.5816	$150.58	$150.60	$150.60
Purchase #5	$125.47	11.669%	$140.1111	$140.11	$140.10	$140.10
Purchase #6	$96.80	6.768%	$103.3514	$103.35	$103.35	$103.40
Purchase #7	$111.31	10.717%	$123.2391	$123.24	$123.25	$123.20
7 purchase totals =	$840.16		$917.9155	$917.91	$917.90	$918.00