I believe rough drafts should be exactly that-rough. You just want your idea to go from head to paper. That's quite a trick in and of itself- to put words to the images that appear in your mind, to somehow wrestle all of that action onto a still, piece of paper. So, my advice is always to just write it. It will be bad-very bad- but it's a rough draft and it's supposed to be bad. You can fix it once it's on the paper.
I do try to follow my own advice. But for me the beginnings are different. The opening line and beginning paragraph have to be perfect. I can spend weeks, and in the case of this winter, months writing that opening paragraph. Why is that?
I think the opening paragraph sets the tone. However the language flows in the beginning is how it will flow throughout the story. The voice must be clear, and until that voice is firmly and distinctly established in the opening paragraph, it's difficult for me to proceed to the story. Now, once I have that opening paragraph in place complete with tone and voice, I can dash off the rest of the story in its roughest form, knowing that I'll go back and rewrite it many, many times.
The beginning paragraph is like a key to a secret gate. You have to spend time finding exactly the right key, and once you turn it in the lock, you can step through the gate and discover what's behind the wall.