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	$24.73	11.760%	$27.6382	$27.64	$27.65	$27.60
Purchase #2	$225.27	10.460%	$248.8332	$248.83	$248.85	$248.80
Purchase #3	$57.66	11.834%	$64.4835	$64.48	$64.50	$64.50
Purchase #4	$170.28	11.725%	$190.2453	$190.25	$190.25	$190.20
Purchase #5	$26.42	3.271%	$27.2842	$27.28	$27.30	$27.30
Purchase #6	$186.86	7.331%	$200.5587	$200.56	$200.55	$200.60
Purchase #7	$129.36	8.826%	$140.7773	$140.78	$140.80	$140.80
7 purchase totals =	$820.58		$899.8204	$899.82	$899.90	$899.80