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	$37.59	11.734%	$42.0008	$42.00	$42.00	$42.00
Purchase #2	$6.86	8.852%	$7.4672	$7.47	$7.45	$7.50
Purchase #3	$93.65	10.424%	$103.4121	$103.41	$103.40	$103.40
Purchase #4	$63.39	7.763%	$68.311	$68.31	$68.30	$68.30
Purchase #5	$94.99	10.423%	$104.8908	$104.89	$104.90	$104.90
Purchase #6	$213.44	6.344%	$226.9806	$226.98	$227.00	$227.00
Purchase #7	$62.89	7.814%	$67.8042	$67.80	$67.80	$67.80
7 purchase totals =	$572.81		$620.8667	$620.86	$620.85	$620.90