what do you consider to be a classic Manhattan?

Rye, (not bourbon) sweet vermouth, an gostura bitters, garnished with a cherry. I would say that is a classic manhattan. Although I do drink them either with Rye or Bourbon

A classic Manhattan will not have the maraschino cherry juice in it even though you are muddling maraschino cherries, an orange slice and bitters together. Purists insist on not having the cherry juice, but the bartender I work with does use it and it fools even the most ardent purist.

So as long as you have a perfect balance between the bourbon and sweet vermouth, you may not need it, but it always comes down to the flavor and if you can make a good cocktail.