There is a number of ivy spots and they are all pretty much the same. I see lots of people at the ivy behind varrock castle, and I know my little brother got 99 woodcut at the Ivy on wall between Falador and Taverly.
Just pick a spot and go, go, go! Like the guy above me said, make sure you got alot of music, movies to watch etc. as it is going to be one mind numbing, robotically repetitive task.