There are lots of other measures you can (and need) to take to protect your dog. It's a long list, but most folks would agree that parking in the shade and keeping the dog hydrated are solid tips. Others will also chime in about letting the dog cool off before putting it back in the box and making sure the box is adequately ventilated.
So, the answer is NO...fans are not needed, until they are needed...and then you do need them
Mr. Ounce of Prevention