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	$13.45	13.300%	$15.2389	$15.24	$15.25	$15.20
Purchase #2	$219.48	9.429%	$240.1748	$240.17	$240.15	$240.20
Purchase #3	$74.34	8.260%	$80.4805	$80.48	$80.50	$80.50
Purchase #4	$106.35	11.600%	$118.6866	$118.69	$118.70	$118.70
Purchase #5	$29.30	7.460%	$31.4858	$31.49	$31.50	$31.50
Purchase #6	$95.54	12.679%	$107.6535	$107.65	$107.65	$107.70
Purchase #7	$67.72	7.343%	$72.6927	$72.69	$72.70	$72.70
7 purchase totals =	$606.18		$666.4128	$666.41	$666.45	$666.50