Wikipedia's article on fashion during the period has this to say about men's hair: Wigs were worn for formal occasions, or the hair was worn long and powdered, brushed back from the forehead and clubbed (tied back at the nape of the neck) with a black ribbon.
http://en.wikipedia.org/wiki/1750-1795_in_fashion#Hairstyles_and_headgear_2
Here are some portraits of a few of the more famous founders - and the others also appear to have similar non-parting looks?

George Washington

Ben Franklin

John Hancock

Alexander Hamliton


Thomas Jefferson

John Adams