Two brads are 10 seconds quicker, but what does that matter to a good story?
True. But consider the following...
You are a tired reader who think he/she knows everything about screenwriting after taking McKee's weekend seminar or maybe is in his/her second semester at UCLA and has a pile of fifty scripts to go through by the weekend.
You see a script with bad formatting or long tracts of action or dialog (or any other things that may be considered taboo or a sign of a newbie - maybe even the use of those screws) and thinks to yourself
"this dude obviously knows nothing. It probably sucks." and tosses it into the bottom of the pile-- or worse, doesn't read it at all and fills out the report card by guessing.
Now of course, using two versus three brads isn't much of a indiscretion (two brads are considered the pro way by the way) and most certainly will not taint your chances with the know-it-all-reader, but don't take chances because they won't know it's a good story until the get to the last page and anything (I mean anything) that may keep them from getting to the last page or not wanting to, ruins your chances.
I only mention this because I hear a lot of people with bad skills saying "It's a great story," but they fail to understand that you need to read it to see it. Do not fall into bad habits because you think your story is great.
This is true for comps as well as studio/producer readers. A book, or screenplay, is many times judged by its cover.
Funny story though. The guy that won Nicholls for "Finding Forester" was asked by the fellowship committee to fix the typos and formatting problems in his submission, after winning, so the Academy would have a "clean and perfect" copy for the archives. So, I guess they over looked those issues when judging and the initial reader wasn't a jerk and actually read the script to the end, but imagine if the initial reader was a jerk, like so many of them are. Sean Connery would have had one less credit to his name.
To answer your questions, send it early, but not within the first month of accepting submissions. I like to send right in the middle of the timeline if possible. Just don't send it in the last possible second.
Also, do not rely on spell checkers or grammar checkers. If you have any doubts, go to a proof reader and spend a $100 bucks or so to have it professionally proofed. If their are none in your area or you do not want to use the host of online proofers (google search) then call up your local newspaper and ask for advice. My proofer is a NY times copywriter.