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	$6.92	11.516%	$7.7169	$7.72	$7.70	$7.70
Purchase #2	$214.48	13.680%	$243.8209	$243.82	$243.80	$243.80
Purchase #3	$78.39	9.507%	$85.8425	$85.84	$85.85	$85.80
Purchase #4	$49.37	3.384%	$51.0407	$51.04	$51.05	$51.00
Purchase #5	$51.67	5.190%	$54.3517	$54.35	$54.35	$54.40
Purchase #6	$51.39	10.344%	$56.7058	$56.71	$56.70	$56.70
Purchase #7	$90.53	11.612%	$101.0423	$101.04	$101.05	$101.00
7 purchase totals =	$542.75		$600.5208	$600.52	$600.50	$600.40