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	$38.31	9.798%	$42.0636	$42.06	$42.05	$42.10
Purchase #2	$171.61	11.848%	$191.9424	$191.94	$191.95	$191.90
Purchase #3	$93.32	12.648%	$105.1231	$105.12	$105.10	$105.10
Purchase #4	$84.56	4.642%	$88.4853	$88.49	$88.50	$88.50
Purchase #5	$242.97	4.307%	$253.4347	$253.43	$253.45	$253.40
Purchase #6	$56.92	9.362%	$62.2489	$62.25	$62.25	$62.20
Purchase #7	$45.75	5.386%	$48.2141	$48.21	$48.20	$48.20
7 purchase totals =	$733.44		$791.5121	$791.50	$791.50	$791.40