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	$38.61	8.800%	$42.0077	$42.01	$42.00	$42.00
Purchase #2	$146.43	13.584%	$166.3211	$166.32	$166.30	$166.30
Purchase #3	$228.85	7.486%	$245.9817	$245.98	$246.00	$246.00
Purchase #4	$38.37	10.687%	$42.4706	$42.47	$42.45	$42.50
Purchase #5	$44.33	7.227%	$47.5337	$47.53	$47.55	$47.50
Purchase #6	$32.56	13.179%	$36.8511	$36.85	$36.85	$36.90
Purchase #7	$26.60	7.781%	$28.6697	$28.67	$28.65	$28.70
7 purchase totals =	$555.75		$609.8356	$609.83	$609.80	$609.90