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	$18.84	12.763%	$21.2445	$21.24	$21.25	$21.20
Purchase #2	$119.81	13.517%	$136.0047	$136.00	$136.00	$136.00
Purchase #3	$188.40	8.610%	$204.6212	$204.62	$204.60	$204.60
Purchase #4	$165.68	7.705%	$178.4456	$178.45	$178.45	$178.40
Purchase #5	$238.80	6.593%	$254.5441	$254.54	$254.55	$254.50
Purchase #6	$156.72	3.660%	$162.456	$162.46	$162.45	$162.50
Purchase #7	$153.51	8.529%	$166.6029	$166.60	$166.60	$166.60
7 purchase totals =	$1,041.76		$1,123.9190	$1,123.91	$1,123.90	$1,123.80