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	$153.92	12.728%	$173.5109	$173.51	$173.50	$173.50
Purchase #2	$234.47	12.790%	$264.4587	$264.46	$264.45	$264.50
Purchase #3	$104.85	10.310%	$115.66	$115.66	$115.65	$115.70
Purchase #4	$164.46	10.841%	$182.2891	$182.29	$182.30	$182.30
Purchase #5	$78.66	5.293%	$82.8235	$82.82	$82.80	$82.80
Purchase #6	$105.64	13.654%	$120.0641	$120.06	$120.05	$120.10
Purchase #7	$48.47	5.462%	$51.1174	$51.12	$51.10	$51.10
7 purchase totals =	$890.47		$989.9237	$989.92	$989.85	$990.00