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	$39.41	6.870%	$42.1175	$42.12	$42.10	$42.10
Purchase #2	$34.94	6.815%	$37.3212	$37.32	$37.30	$37.30
Purchase #3	$140.94	6.478%	$150.0701	$150.07	$150.05	$150.10
Purchase #4	$172.64	6.577%	$183.9945	$183.99	$184.00	$184.00
Purchase #5	$102.94	5.372%	$108.4699	$108.47	$108.45	$108.50
Purchase #6	$196.47	7.386%	$210.9813	$210.98	$211.00	$211.00
Purchase #7	$232.89	7.533%	$250.4336	$250.43	$250.45	$250.40
7 purchase totals =	$920.23		$983.3881	$983.38	$983.35	$983.40