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	$39.79	6.608%	$42.4193	$42.42	$42.40	$42.40
Purchase #2	$104.96	3.716%	$108.8603	$108.86	$108.85	$108.90
Purchase #3	$31.56	6.870%	$33.7282	$33.73	$33.75	$33.70
Purchase #4	$240.72	8.789%	$261.8769	$261.88	$261.90	$261.90
Purchase #5	$49.82	12.446%	$56.0206	$56.02	$56.00	$56.00
Purchase #6	$216.52	13.811%	$246.4236	$246.42	$246.40	$246.40
Purchase #7	$96.78	12.815%	$109.1824	$109.18	$109.20	$109.20
7 purchase totals =	$780.15		$858.5113	$858.51	$858.50	$858.50