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	$37.74	5.619%	$39.8606	$39.86	$39.85	$39.90
Purchase #2	$13.42	3.821%	$13.9328	$13.93	$13.95	$13.90
Purchase #3	$38.88	4.673%	$40.6969	$40.70	$40.70	$40.70
Purchase #4	$85.76	3.669%	$88.9065	$88.91	$88.90	$88.90
Purchase #5	$166.87	5.795%	$176.5401	$176.54	$176.55	$176.50
Purchase #6	$103.49	8.167%	$111.942	$111.94	$111.95	$111.90
Purchase #7	$150.27	11.359%	$167.3392	$167.34	$167.35	$167.30
7 purchase totals =	$596.43		$639.2181	$639.22	$639.25	$639.10