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	$37.75	11.705%	$42.1686	$42.17	$42.15	$42.20
Purchase #2	$76.81	4.824%	$80.5153	$80.52	$80.50	$80.50
Purchase #3	$28.89	9.637%	$31.6741	$31.67	$31.65	$31.70
Purchase #4	$70.82	10.303%	$78.1166	$78.12	$78.10	$78.10
Purchase #5	$66.92	8.707%	$72.7467	$72.75	$72.75	$72.70
Purchase #6	$78.68	12.572%	$88.5716	$88.57	$88.55	$88.60
Purchase #7	$244.43	13.209%	$276.7168	$276.72	$276.70	$276.70
7 purchase totals =	$604.30		$670.5097	$670.52	$670.40	$670.50