I have no experience with screenwriting specifically, but I see no reason why this would be different from the considerations in the fiction market (where "sending chapters" is always the first three chapters: the reader will start from the beginning, precisely like a viewer would. Why would he do otherwise? That's what makes sense; that's what gives him the best understanding and experience of the work as a whole.
But beginnings are naturally slow and less-exciting? True. Also: tough. Quite simply, there's no good excuse for having a dull beginning. As a screenwriter, it's your job to bring the entire script up to par - before submission. That means you need to overcome the "how do I draw viewers into the movie" problem, just like later on you'll need to overcome the typical problems of a screenplay's middle and end.
Don't think of it as looking for a "sample of the writing," because that's not really what it is. It's "I'm going to experience this screenplay precisely as a viewer would, but if at any point I'm persuaded that it's not good enough, then I can stop." In a vast majority of cases, that point comes pretty early.
That being said, readers know movies and their genres. You don't need to start out with an erupting volcano because the opening of your domestic dramady isn't exciting enough. Lots of movies open with endearing characters, or great settings, or fun dialogue, or whatever. If you know why you would be riveted to this movie from the start, why your expected audience would leap right into this movie as soon as it begins, then you're absolutely fine. But if everybody would be bored by the first 15 minutes before "things get interesting," then that's a real problem - and the reader's quite right to reject your script as unready.