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	$129.66	13.361%	$146.9839	$146.98	$147.00	$147.00
Purchase #2	$34.76	11.499%	$38.7571	$38.76	$38.75	$38.80
Purchase #3	$18.53	12.412%	$20.8299	$20.83	$20.85	$20.80
Purchase #4	$90.95	13.820%	$103.5193	$103.52	$103.50	$103.50
Purchase #5	$174.33	9.116%	$190.2219	$190.22	$190.20	$190.20
Purchase #6	$236.72	12.299%	$265.8342	$265.83	$265.85	$265.80
Purchase #7	$32.61	3.866%	$33.8707	$33.87	$33.85	$33.90
7 purchase totals =	$717.56		$800.0170	$800.01	$800.00	$800.00