I think partly it depends what the method/topic is, which is why some unis don't give a minimum word count for FYPs/dissertations (e.g. if your method is very complex it will require a much higher word count than a very simple method).
I've just handed in my Final Year Project which had a max. word count of 6,000, and mine is 4,900. Can't say if it will get a good mark yet, but on the parts that my supervisor looked at there were no major issues. I've always had a tendency to write 'very short, concise essays' as soon as I have no word count though, so it works out for me like that.
Saying that, as hobnob said, try to avoid it if at all possible - don't run the risk of missing out something important that you could have discussed/evaluated further. Good luck!