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	$250.97	11.752%	$280.464	$280.46	$280.45	$280.50
Purchase #2	$110.49	5.470%	$116.5338	$116.53	$116.55	$116.50
Purchase #3	$34.34	8.246%	$37.1717	$37.17	$37.15	$37.20
Purchase #4	$136.59	6.271%	$145.1556	$145.16	$145.15	$145.20
Purchase #5	$131.79	9.449%	$144.2428	$144.24	$144.25	$144.20
Purchase #6	$54.89	11.506%	$61.2056	$61.21	$61.20	$61.20
Purchase #7	$67.25	7.398%	$72.2252	$72.23	$72.25	$72.20
7 purchase totals =	$786.32		$856.9987	$857.00	$857.00	$857.00