You should never be contemplating the dialogue (or any other part of the script) during it's first draft writing process anyway..
As Viki King says in her book
How to write a movie in 21 days, "Write from the heart, REwrite from the head"
Simply meaning, don't think about what you write while you write it, and don't go back to change anything until you're finished writing. When you start working on the rewrites, that's when you can be analytical and figure out if the scenes are in the right order, if her hair should really be auburn instead of blonde, if he should really tell her to go F-ck herself or just slam the door in her face, whether or not their discussion about the flavor of coffee was helping to drive the story or just extra 'fluff'.. most (if not all) of that fluff can go away.
Sid Field mentions something in at least one of his screenwriting books about "Whitespace". In which he's talking about the contrast of words to blank page in a script. As a script reader, he said it's a lot easier to read through one with more white per page than other scripts. Even if it is only a psychological thing... cutting the words on the page will help you sell a script, afterall if it doesn't make it past the readers desk you're not going to get anything but a reserved space in the "circular file"
Pixel touched on something that is KEY to writing dialogue. It MUST be necessary to the story. There just simply is not room on the page, or in the script as a whole, for unneeded words that cut down the whitespace and thereby cut down your chances of sale.
Now, on the other hand, if it is a script you're planning to shoot yourself, then even the formatting of it isn't as important, let alone if there's extra unnecessary dialogue or not. That would be where a stage reading may be most helpful, because in this case you're not interested in impressing hollywood people enough to get them to pony up some dough for your story so you can go buy a doughnut and coffee and brag about how you just sold the next hit hollywood movie. You are interested in whether or not the dialogue "works" ... not just to move the story forward, but in general, does it sound natural, is it in line with how that character expresses themself vocally. Personally I still feel that keeping the dialogue to that which is essential for the story is the best bet, but not quite as necessary for a film you plan to shoot yourself.
Why? You don't need to impress anyone so they'll buy it, it's already going to be made, by you! If, during the process of pre-production, production, or post production, you come upon a bit of dialogue that isn't needed, pull it from the script, don't shoot that segment, or edit it out (depending on which stage you're in). It's done all the time, which is a big part of why there are deleted scenes on most DVDs these days. Hey, if you can cut a WHOLE SCENE and still get the story across without confusion, cool! Production just sped up by a few hours to a day or so, congrats, eat a twinkie.
The bottom line is this: Always get into a scene at the last possible moment, and leave it as soon as you can, and make sure dialogue moves the story forward, while also not explaining what can just as easily be shown. If dialogue still troubles you, and you have access to some friends or preferably actors (or better yet, improv people) get them together, explain the scene, and tell them to act it out. Tape these improv sessions, and straight up steal their lines. Why? You're looking for how people talk, right? Well... they just put themselves into your scene, and spoke how they would speak, because they were working without lines.
Sure, they might need to be tweaked a bit, but at least it should help for a scene you just can't quite get the dialogue to work in. Even if you have dialogue written for that scene, I'd say give them the scene breakdown, what they're each trying to accomplish, etc.. but not the dialogue, then let 'em loose and incorporate their dialogue with your scene. If they just totally skip saying something that's needed, write it in, but if they're any good at improv acting and have an idea where the scene needs to go that shouldn't be necessary.
CLEARLY THERE IS NOT ENOUGH WHITESPACE IN THIS POST!