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	$37.60	6.296%	$39.9673	$39.97	$39.95	$40.00
Purchase #2	$38.59	12.452%	$43.3952	$43.40	$43.40	$43.40
Purchase #3	$155.76	6.747%	$166.2691	$166.27	$166.25	$166.30
Purchase #4	$241.80	11.612%	$269.8778	$269.88	$269.90	$269.90
Purchase #5	$132.70	11.632%	$148.1357	$148.14	$148.15	$148.10
Purchase #6	$77.98	4.185%	$81.2435	$81.24	$81.25	$81.20
Purchase #7	$28.72	12.293%	$32.2505	$32.25	$32.25	$32.30
7 purchase totals =	$713.15		$781.1391	$781.15	$781.15	$781.20