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	$10.50	13.500%	$11.9175	$11.92	$11.90	$11.90
Purchase #2	$148.47	10.841%	$164.5656	$164.57	$164.55	$164.60
Purchase #3	$158.60	13.286%	$179.6716	$179.67	$179.65	$179.70
Purchase #4	$6.28	6.638%	$6.6969	$6.70	$6.70	$6.70
Purchase #5	$33.31	5.163%	$35.0298	$35.03	$35.05	$35.00
Purchase #6	$81.86	9.417%	$89.5688	$89.57	$89.55	$89.60
Purchase #7	$86.28	3.732%	$89.5	$89.50	$89.50	$89.50
7 purchase totals =	$525.30		$576.9502	$576.96	$576.90	$577.00