So far I've been including a couple of lines the first time I show a certain location that describes it.  How the room is built, how messy it is, etc.  Is this recommended?  On the one hand it would help the reader understand the scene, but on the other hand it might be unnecessary and distorts the one-page-is-one-minute idea.

Not if it's important to the scene it won't distort it. Why not give a general description of your location and or surrounding area?

I mean... lets say you wanna invoke a certain feel to the reader or producer/agent who might be making this script into a movie...

Would you describe a run down, desensitized, part of town as such --

A downtown shopping district.

or

A downtown shopping disctrict. It's streets littered with trash and broken down cars. Vagrants and homeless people stagger about. Bars cover most of the store front windows.

Or

You could opt for the easy way out and assume we all know what a seedy part of town looks like and go this route.

A seedy part of town.

It's really up to you. However, I always throw in a few lines of descriptions just so I know the reader is feeling what vibe I'm sending out.

Just me.

I only describe locations when there is something special about them which contributes to move the plot forward, or to the tone of the scene. If a certain scene takes place inside an ordinary bedroom, I think "INT. BEDROOM" is enough.

But if it is important to the story to mention that (for example) this bedroom if a complete mess (like my character´s life), then I would mention that there are some slices of pizza on the floor, or whatever. Is not neccesary to describe every detail, just mention a representative detail, by which the reader can figure out the rest.

Nobody cares if there is a picture in the living room. But if your main character is going to steal it in the next scene, then mention it.

Usually when you enter a location, unless you're establishing it, you don't describe much of anything that isn't vital to the flow of the story. You just describe the action and show the characters interacting with whatever is in the room. If they don't interact with it, it's usually not important enough to mention.

If you're establishing it, unless there is a vital piece of information to the story, go with the stereotype location descriptions.

Film is a visual medium. As a screenwriter, it's your job to help us visualise the scene so I believe descriptions of locations are necessary. Keep descriptions as short and tight as possible but give us enough to conjour up a vivid image.

Balt's example is a good one. Are the vagrants, homeless people and bars vital to the story? Probably not. Do they create an atmosphere? Yes.

But there is a difference in creating an atmosphere and getting carried away with it. Vagrants, homeless people, and bars fall into a particular sterotype of a location, and it should be described with some detail, but also with minimalism. You may want to name A bar, but not necessarily all of them. You might mention the vagrants and vagabonds, but it would not be necessary or even wanted to describe their clothing unless it factors into the story. If you say a house has flowers, you don't say what varieties unless it is important to the story. That's kind of what I was going for with stereotype location descriptions.