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	$35.49	11.638%	$39.6203	$39.62	$39.60	$39.60
Purchase #2	$45.64	9.802%	$50.1136	$50.11	$50.10	$50.10
Purchase #3	$35.28	10.432%	$38.9604	$38.96	$38.95	$39.00
Purchase #4	$160.34	8.820%	$174.482	$174.48	$174.50	$174.50
Purchase #5	$216.29	3.485%	$223.8277	$223.83	$223.85	$223.80
Purchase #6	$109.98	12.235%	$123.4361	$123.44	$123.45	$123.40
Purchase #7	$17.71	5.826%	$18.7418	$18.74	$18.75	$18.70
7 purchase totals =	$620.73		$669.1819	$669.18	$669.20	$669.10