I hate to sound defiant, but doesn't it really depend on the story? I mean, if you have elements that are crucial to the plot would you leave them out just to pull your work under the 70,000 mark? Or would you go back and inject so much babble and filler that it made the story incoherent just to make it over the 80,000 mark?
Seems that word count is a far less important issue than content to me. I had an agent ask me to remove 15,000 words from a MS after the query process had begun (which they later turned down). I could only remove about 7000 before the story began to suffer. Seemed an odd rule at the time and it still does.