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	$122.32	12.841%	$138.0271	$138.03	$138.05	$138.00
Purchase #2	$196.40	13.720%	$223.3461	$223.35	$223.35	$223.30
Purchase #3	$49.67	9.174%	$54.2267	$54.23	$54.25	$54.20
Purchase #4	$215.95	7.671%	$232.5155	$232.52	$232.50	$232.50
Purchase #5	$38.72	3.602%	$40.1147	$40.11	$40.10	$40.10
Purchase #6	$171.72	10.515%	$189.7764	$189.78	$189.80	$189.80
Purchase #7	$75.72	9.518%	$82.927	$82.93	$82.95	$82.90
7 purchase totals =	$870.50		$960.9335	$960.95	$961.00	$960.80