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	$24.54	12.239%	$27.5435	$27.54	$27.55	$27.50
Purchase #2	$82.45	13.225%	$93.354	$93.35	$93.35	$93.40
Purchase #3	$210.61	3.664%	$218.3268	$218.33	$218.35	$218.30
Purchase #4	$100.84	10.864%	$111.7953	$111.80	$111.80	$111.80
Purchase #5	$126.31	6.516%	$134.5404	$134.54	$134.55	$134.50
Purchase #6	$19.31	13.584%	$21.9331	$21.93	$21.95	$21.90
Purchase #7	$31.53	4.363%	$32.9057	$32.91	$32.90	$32.90
7 purchase totals =	$595.59		$640.3988	$640.40	$640.45	$640.30