This can be represented by a simple formula:
Allowability = (Plot Device Awesome - Real Science Aweseome)/ Badness of the science
What this boils down to is, if your plot device is very awesome, and your science isn't too bad go for it. If, on the other hand, your plot device is kinda underwhelming, and/or the amount of bad science required to realize it is very high, then probably come up with a different plot device.
I'll give an example of each so you can see what I'm talking about. Let's start out with the bad science that doesn't in any way contribute to the story. What we are going to use is one of the worst science movies of all time, Mission to Mars. The scene which is the most guilty of this is the scene where the astronauts are transferring from their damaged space craft to an orbiting supply ship. Tim Robbins' character overshot the supply ship and was drifting off into space. His wife sets up her suit to use half of its fuel to try to thrust out to him and save him. Right there we have our problem. If you use half your fuel to thrust away from something in space, it takes the other half to stop. What they did was allow her to thrust away from the ship and then magically stop in space for her to be able to turn around and live! Let's try running this through our equation.
(Bad plot device - Both of them die ending the movie sooner!)/Terrible Scene = Shouldn't have been used.
Now, to help us feel better, let's give an example of a really good scene. The new Star Trek movie, love it or hate it, this movie had good science. Now, if you've seen this movie, you're probably wondering what I found to pick on. I admit, I didn't find this, it was pointed out to me by the most excellent Phil Plait. There was a scene where the Enterprise was coming up out of Titan's atmosphere to to ambush the Romulans. I know what you are thinking, this scene had some great science, Titan's atmosphere was the correct color, the Saturn images looked straight out of Cassini, what's the problem? Answer, Titan's orbit is in the same plane as the rings, you wouldn't be able to see them from Titan. However, let's just run that through the equation for this scene:
(Awesome view of Saturn's rings - Not seeing Saturn's rings)/Awesome Dramatic Scene = Must include in movie.
So, as you can see, even if the science is a little bad, you can use it in movies. Just try not to make the science types out there cringe.