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	$19.51	7.522%	$20.9775	$20.98	$21.00	$21.00
Purchase #2	$59.82	12.193%	$67.1139	$67.11	$67.10	$67.10
Purchase #3	$55.43	12.124%	$62.1503	$62.15	$62.15	$62.20
Purchase #4	$58.58	5.571%	$61.8435	$61.84	$61.85	$61.80
Purchase #5	$96.76	13.340%	$109.6678	$109.67	$109.65	$109.70
Purchase #6	$57.94	10.217%	$63.8597	$63.86	$63.85	$63.90
Purchase #7	$54.31	12.378%	$61.0325	$61.03	$61.05	$61.00
7 purchase totals =	$402.35		$446.6452	$446.64	$446.65	$446.70