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	$7.45	12.861%	$8.4081	$8.41	$8.40	$8.40
Purchase #2	$37.39	12.353%	$42.0088	$42.01	$42.00	$42.00
Purchase #3	$168.55	13.804%	$191.8166	$191.82	$191.80	$191.80
Purchase #4	$170.69	13.268%	$193.3371	$193.34	$193.35	$193.30
Purchase #5	$223.98	4.140%	$233.2528	$233.25	$233.25	$233.30
Purchase #6	$103.49	6.749%	$110.4745	$110.47	$110.45	$110.50
Purchase #7	$56.29	6.446%	$59.9185	$59.92	$59.90	$59.90
7 purchase totals =	$767.84		$839.2164	$839.22	$839.15	$839.20