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	$134.89	3.583%	$139.7231	$139.72	$139.70	$139.70
Purchase #2	$124.60	4.591%	$130.3204	$130.32	$130.30	$130.30
Purchase #3	$224.31	8.749%	$243.9349	$243.93	$243.95	$243.90
Purchase #4	$40.79	12.473%	$45.8777	$45.88	$45.90	$45.90
Purchase #5	$36.89	6.858%	$39.4199	$39.42	$39.40	$39.40
Purchase #6	$239.48	12.327%	$269.0007	$269.00	$269.00	$269.00
Purchase #7	$10.39	8.108%	$11.2324	$11.23	$11.25	$11.20
7 purchase totals =	$811.35		$879.5091	$879.50	$879.50	$879.40