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	$33.69	12.761%	$37.9892	$37.99	$38.00	$38.00
Purchase #2	$184.65	6.431%	$196.5248	$196.52	$196.50	$196.50
Purchase #3	$142.47	4.194%	$148.4452	$148.45	$148.45	$148.40
Purchase #4	$71.61	12.145%	$80.307	$80.31	$80.30	$80.30
Purchase #5	$220.99	12.305%	$248.1828	$248.18	$248.20	$248.20
Purchase #6	$205.73	8.189%	$222.5772	$222.58	$222.60	$222.60
Purchase #7	$44.92	12.581%	$50.5714	$50.57	$50.55	$50.60
7 purchase totals =	$904.06		$984.5976	$984.60	$984.60	$984.60