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	$7.67	9.308%	$8.3839	$8.38	$8.40	$8.40
Purchase #2	$199.79	4.180%	$208.1412	$208.14	$208.15	$208.10
Purchase #3	$45.54	11.584%	$50.8154	$50.82	$50.80	$50.80
Purchase #4	$224.78	11.399%	$250.4027	$250.40	$250.40	$250.40
Purchase #5	$31.41	12.828%	$35.4393	$35.44	$35.45	$35.40
Purchase #6	$150.33	6.363%	$159.8955	$159.90	$159.90	$159.90
Purchase #7	$183.34	8.819%	$199.5088	$199.51	$199.50	$199.50
7 purchase totals =	$842.86		$912.5868	$912.59	$912.60	$912.50