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	$123.47	13.207%	$139.7767	$139.78	$139.80	$139.80
Purchase #2	$199.75	3.241%	$206.2239	$206.22	$206.20	$206.20
Purchase #3	$162.79	3.286%	$168.1393	$168.14	$168.15	$168.10
Purchase #4	$68.61	11.240%	$76.3218	$76.32	$76.30	$76.30
Purchase #5	$53.31	5.611%	$56.3012	$56.30	$56.30	$56.30
Purchase #6	$132.62	10.279%	$146.252	$146.25	$146.25	$146.30
Purchase #7	$42.51	10.282%	$46.8809	$46.88	$46.90	$46.90
7 purchase totals =	$783.06		$839.8958	$839.89	$839.90	$839.90