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	$102.88	11.638%	$114.8532	$114.85	$114.85	$114.90
Purchase #2	$112.99	12.858%	$127.5183	$127.52	$127.50	$127.50
Purchase #3	$90.83	8.459%	$98.5133	$98.51	$98.50	$98.50
Purchase #4	$34.33	10.753%	$38.0215	$38.02	$38.00	$38.00
Purchase #5	$122.94	6.740%	$131.2262	$131.23	$131.25	$131.20
Purchase #6	$134.60	4.807%	$141.0702	$141.07	$141.05	$141.10
Purchase #7	$170.91	10.449%	$188.7684	$188.77	$188.75	$188.80
7 purchase totals =	$769.48		$839.9711	$839.97	$839.90	$840.00