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	$16.60	13.605%	$18.8584	$18.86	$18.85	$18.90
Purchase #2	$206.45	10.109%	$227.32	$227.32	$227.30	$227.30
Purchase #3	$125.77	12.779%	$141.8421	$141.84	$141.85	$141.80
Purchase #4	$87.62	6.501%	$93.3162	$93.32	$93.30	$93.30
Purchase #5	$135.68	7.747%	$146.1911	$146.19	$146.20	$146.20
Purchase #6	$71.87	9.768%	$78.8903	$78.89	$78.90	$78.90
Purchase #7	$171.41	5.784%	$181.3244	$181.32	$181.30	$181.30
7 purchase totals =	$815.40		$887.7425	$887.74	$887.70	$887.70