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	$37.29	12.863%	$42.0866	$42.09	$42.10	$42.10
Purchase #2	$77.46	8.435%	$83.9938	$83.99	$84.00	$84.00
Purchase #3	$16.39	8.807%	$17.8335	$17.83	$17.85	$17.80
Purchase #4	$165.98	5.353%	$174.8649	$174.86	$174.85	$174.90
Purchase #5	$34.88	9.284%	$38.1183	$38.12	$38.10	$38.10
Purchase #6	$194.67	12.787%	$219.5625	$219.56	$219.55	$219.60
Purchase #7	$82.78	8.164%	$89.5382	$89.54	$89.55	$89.50
7 purchase totals =	$609.45		$665.9978	$665.99	$666.00	$666.00