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	$36.48	8.252%	$39.4903	$39.49	$39.50	$39.50
Purchase #2	$34.71	12.159%	$38.9304	$38.93	$38.95	$38.90
Purchase #3	$135.71	4.601%	$141.954	$141.95	$141.95	$142.00
Purchase #4	$35.34	13.179%	$39.9975	$40.00	$40.00	$40.00
Purchase #5	$143.52	9.247%	$156.7913	$156.79	$156.80	$156.80
Purchase #6	$38.39	5.841%	$40.6324	$40.63	$40.65	$40.60
Purchase #7	$128.31	9.156%	$140.0581	$140.06	$140.05	$140.10
7 purchase totals =	$552.46		$597.8540	$597.85	$597.90	$597.90