I'd say that running a good PFT is two-fold physical/mental. If you're feeling energized and well rested you're gonna just do that much better. Tell your OSO you need a few days, I'm sure they'll be understanding and will accomodate you.
I ran it one time with only a few hours of sleep and a nasty hangover.. still turned out with a 278!!! Mind over matter... but believe you me if it wasn't last minute I would have asked my OSO for a day or two, I felt like the world was coming to an end after that run.
Just remember that you want to go to OCS with as much endurance as possible... you're body is inevitably going to get broken down, and you'll probably get sick at least once. The more endurance you ship with, the better your body will hold up against sick candidates, pathogens, quigly goo, etc. Remember, you can take the candidate out of the quigly, but you can't take the quigly out of the candidate...you'll see what I mean.