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	$249.42	12.259%	$279.9964	$280.00	$280.00	$280.00
Purchase #2	$237.59	11.867%	$265.7848	$265.78	$265.80	$265.80
Purchase #3	$240.39	3.808%	$249.5441	$249.54	$249.55	$249.50
Purchase #4	$150.32	7.302%	$161.2964	$161.30	$161.30	$161.30
Purchase #5	$226.50	11.383%	$252.2825	$252.28	$252.30	$252.30
Purchase #6	$104.59	12.358%	$117.5152	$117.52	$117.50	$117.50
Purchase #7	$79.82	13.835%	$90.8631	$90.86	$90.85	$90.90
7 purchase totals =	$1,288.63		$1,417.2825	$1,417.28	$1,417.30	$1,417.30