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	$39.39	6.631%	$42.002	$42.00	$42.00	$42.00
Purchase #2	$161.89	6.123%	$171.8025	$171.80	$171.80	$171.80
Purchase #3	$136.76	13.820%	$155.6602	$155.66	$155.65	$155.70
Purchase #4	$129.34	11.640%	$144.3952	$144.40	$144.40	$144.40
Purchase #5	$38.91	6.592%	$41.4749	$41.47	$41.45	$41.50
Purchase #6	$42.54	12.594%	$47.8975	$47.90	$47.90	$47.90
Purchase #7	$33.86	11.663%	$37.8091	$37.81	$37.80	$37.80
7 purchase totals =	$582.69		$641.0414	$641.04	$641.00	$641.10