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	$38.39	9.660%	$42.0985	$42.10	$42.10	$42.10
Purchase #2	$222.98	10.478%	$246.3438	$246.34	$246.35	$246.30
Purchase #3	$229.77	12.128%	$257.6365	$257.64	$257.65	$257.60
Purchase #4	$144.92	3.575%	$150.1009	$150.10	$150.10	$150.10
Purchase #5	$148.35	11.482%	$165.3835	$165.38	$165.40	$165.40
Purchase #6	$87.81	9.473%	$96.1282	$96.13	$96.15	$96.10
Purchase #7	$28.48	13.472%	$32.3168	$32.32	$32.30	$32.30
7 purchase totals =	$900.70		$990.0082	$990.01	$990.05	$989.90