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	$17.92	3.211%	$18.4954	$18.50	$18.50	$18.50
Purchase #2	$196.96	12.414%	$221.4106	$221.41	$221.40	$221.40
Purchase #3	$109.74	3.620%	$113.7126	$113.71	$113.70	$113.70
Purchase #4	$59.28	6.292%	$63.0099	$63.01	$63.00	$63.00
Purchase #5	$99.89	11.153%	$111.0307	$111.03	$111.05	$111.00
Purchase #6	$156.64	9.283%	$171.1809	$171.18	$171.20	$171.20
Purchase #7	$143.47	12.740%	$161.7481	$161.75	$161.75	$161.70
7 purchase totals =	$783.90		$860.5882	$860.59	$860.60	$860.50