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	$18.88	12.538%	$21.2472	$21.25	$21.25	$21.20
Purchase #2	$234.39	10.491%	$258.9799	$258.98	$259.00	$259.00
Purchase #3	$82.49	10.276%	$90.9667	$90.97	$90.95	$91.00
Purchase #4	$227.87	11.615%	$254.3371	$254.34	$254.35	$254.30
Purchase #5	$13.49	5.233%	$14.1959	$14.20	$14.20	$14.20
Purchase #6	$147.49	11.504%	$164.4572	$164.46	$164.45	$164.50
Purchase #7	$111.85	10.535%	$123.6334	$123.63	$123.65	$123.60
7 purchase totals =	$836.46		$927.8174	$927.83	$927.85	$927.80