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	$29.60	7.556%	$31.8366	$31.84	$31.85	$31.80
Purchase #2	$139.76	13.185%	$158.1874	$158.19	$158.20	$158.20
Purchase #3	$240.59	11.285%	$267.7406	$267.74	$267.75	$267.70
Purchase #4	$188.28	8.490%	$204.265	$204.27	$204.25	$204.30
Purchase #5	$62.95	13.777%	$71.6226	$71.62	$71.60	$71.60
Purchase #6	$28.79	12.873%	$32.4961	$32.50	$32.50	$32.50
Purchase #7	$137.31	10.791%	$152.1271	$152.13	$152.15	$152.10
7 purchase totals =	$827.28		$918.2754	$918.29	$918.30	$918.20