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	$19.52	12.221%	$21.9055	$21.91	$21.90	$21.90
Purchase #2	$246.98	13.218%	$279.6258	$279.63	$279.65	$279.60
Purchase #3	$174.34	3.865%	$181.0782	$181.08	$181.10	$181.10
Purchase #4	$81.25	13.339%	$92.0879	$92.09	$92.10	$92.10
Purchase #5	$19.68	11.584%	$21.9597	$21.96	$21.95	$22.00
Purchase #6	$54.60	8.570%	$59.2792	$59.28	$59.30	$59.30
Purchase #7	$13.53	3.167%	$13.9585	$13.96	$13.95	$14.00
7 purchase totals =	$609.90		$669.8948	$669.91	$669.95	$670.00