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	$82.33	9.519%	$90.167	$90.17	$90.15	$90.20
Purchase #2	$134.32	3.455%	$138.9608	$138.96	$138.95	$139.00
Purchase #3	$69.71	12.580%	$78.4795	$78.48	$78.50	$78.50
Purchase #4	$36.41	10.245%	$40.1402	$40.14	$40.15	$40.10
Purchase #5	$181.52	5.699%	$191.8648	$191.86	$191.85	$191.90
Purchase #6	$221.37	8.534%	$240.2617	$240.26	$240.25	$240.30
Purchase #7	$122.53	13.426%	$138.9809	$138.98	$139.00	$139.00
7 purchase totals =	$848.19		$918.8549	$918.85	$918.85	$919.00