Yeah, when you start getting up to 5000 or so, I'm not sure it would have to go right up to the limit. For 1000, 1500, 2000, I think those are aiming for a particular "meat" of story, if that makes sense, and you'd be shorting yourself to go substantially less. Like writing a 50-word story for the 75. Surely there's something that could be added to make it a better story, if you have that much room to play with. But one or two words in a 75, or fifty words in a 1000, that probably isn't critical. Still, I'm a fan of using the word count right up to the hilt.
On the other hand, it's important to make sure it doesn't read like it was a 3000 that was cut down to 1000, too. Writing to the limit as well as writing to the limit, as it were.