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	$37.38	12.755%	$42.1478	$42.15	$42.15	$42.10
Purchase #2	$77.52	9.837%	$85.1456	$85.15	$85.15	$85.10
Purchase #3	$205.36	7.672%	$221.1152	$221.12	$221.10	$221.10
Purchase #4	$150.80	10.473%	$166.5933	$166.59	$166.60	$166.60
Purchase #5	$103.73	9.262%	$113.3375	$113.34	$113.35	$113.30
Purchase #6	$240.49	10.298%	$265.2557	$265.26	$265.25	$265.30
Purchase #7	$24.79	10.610%	$27.4202	$27.42	$27.40	$27.40
7 purchase totals =	$840.07		$921.0153	$921.03	$921.00	$920.90