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	$5.54	12.156%	$6.2134	$6.21	$6.20	$6.20
Purchase #2	$211.40	12.565%	$237.9624	$237.96	$237.95	$238.00
Purchase #3	$177.52	4.152%	$184.8906	$184.89	$184.90	$184.90
Purchase #4	$224.77	13.828%	$255.8512	$255.85	$255.85	$255.90
Purchase #5	$132.63	13.273%	$150.234	$150.23	$150.25	$150.20
Purchase #6	$61.77	9.143%	$67.4176	$67.42	$67.40	$67.40
Purchase #7	$20.30	12.570%	$22.8517	$22.85	$22.85	$22.90
7 purchase totals =	$833.93		$925.4209	$925.41	$925.40	$925.50