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	$79.36	12.801%	$89.5189	$89.52	$89.50	$89.50
Purchase #2	$123.48	12.539%	$138.9632	$138.96	$138.95	$139.00
Purchase #3	$77.79	7.110%	$83.3209	$83.32	$83.30	$83.30
Purchase #4	$88.39	8.472%	$95.8784	$95.88	$95.90	$95.90
Purchase #5	$72.73	10.832%	$80.6081	$80.61	$80.60	$80.60
Purchase #6	$48.43	4.118%	$50.4243	$50.42	$50.40	$50.40
Purchase #7	$40.53	8.283%	$43.8871	$43.89	$43.90	$43.90
7 purchase totals =	$530.71		$582.6009	$582.60	$582.55	$582.60