My personal policy is this: If I can't be bothered to watch the entire movie, I won't blam it. If I think what I've watched so far is utter crap, I'll just abstain from voting on that entry unless I watch the whole thing. Yes, it usually is pretty obvious within the first ten seconds or so, but I'll still give a movie a chance.
A good example - I just watched a very badly drawn, poorly animated, poorly voiced movie about a half hour ago. As I was watching it, I was sure I was going to blam it.... And then at the end of the movie, it got REALLY FUNNY... The humor factor outweighed everything else.
If I had watched the first ten seconds of this movie and stopped, I never would have gotten as far as the funny part. This is why I don't feel that you can ever judge a movie by how it looks at first, or how the preloader looks, or whatever.
Similarly, I've watched movies that looked like they were going to be really great - and by the end of the movie all I could think was "Is this thing OVER yet? It's TERRIBLE."
So once again, I state my general belief on this - if you can't be bothered to watch the whole thing, don't vote on it. There's more to life than blam/save points.
Looking at my profile, it looks like I've blammed a little over twice the number of movies I've saved. But then, I take lots of things into consideration, and generally only use zero for total crap, and only use 5 for absolutely amazing.