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	$29.77	7.212%	$31.917	$31.92	$31.90	$31.90
Purchase #2	$139.76	8.132%	$151.1253	$151.13	$151.15	$151.10
Purchase #3	$139.49	12.213%	$156.5259	$156.53	$156.55	$156.50
Purchase #4	$145.59	6.185%	$154.5947	$154.59	$154.60	$154.60
Purchase #5	$46.59	11.378%	$51.891	$51.89	$51.90	$51.90
Purchase #6	$38.44	5.807%	$40.6722	$40.67	$40.65	$40.70
Purchase #7	$184.96	9.306%	$202.1724	$202.17	$202.15	$202.20
7 purchase totals =	$724.60		$788.8985	$788.90	$788.90	$788.90