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	$28.37	9.524%	$31.072	$31.07	$31.05	$31.10
Purchase #2	$243.78	11.829%	$272.6167	$272.62	$272.60	$272.60
Purchase #3	$144.36	5.471%	$152.2579	$152.26	$152.25	$152.30
Purchase #4	$152.60	4.325%	$159.2	$159.20	$159.20	$159.20
Purchase #5	$77.28	12.364%	$86.8349	$86.83	$86.85	$86.80
Purchase #6	$172.88	3.300%	$178.585	$178.59	$178.60	$178.60
Purchase #7	$95.82	4.219%	$99.8626	$99.86	$99.85	$99.90
7 purchase totals =	$915.09		$980.4291	$980.43	$980.40	$980.50