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	$84.71	6.616%	$90.3144	$90.31	$90.30	$90.30
Purchase #2	$117.25	10.604%	$129.6832	$129.68	$129.70	$129.70
Purchase #3	$169.87	9.648%	$186.2591	$186.26	$186.25	$186.30
Purchase #4	$78.38	6.534%	$83.5013	$83.50	$83.50	$83.50
Purchase #5	$39.98	6.648%	$42.6379	$42.64	$42.65	$42.60
Purchase #6	$193.40	5.248%	$203.5496	$203.55	$203.55	$203.50
Purchase #7	$73.32	7.612%	$78.9011	$78.90	$78.90	$78.90
7 purchase totals =	$756.91		$814.8466	$814.84	$814.85	$814.80