I think he was getting at: if you cast upstream, you end up with alot of slack line unless you keep trimming it up and letting it out as it drifts. This makes setting the hook a real quick and tricky prospect; Ive lost a couple bows this way. I'm talking about flies here, not much into floats myself.
However, the 45* sweep keeps the line relatively taught, but does betray the natural presentation. I have found I maintain the most control by rolling it out at an angle, but timing the drift so that the fly crosses my target zone in a natural speed and direction. This means that the fly sweeps into position, then drifts past the target, whether it be a pool or riffle or quel qu'un chose, in such a way that I always have control over the line to some degree.
I would recommend experimenting with some drifting to see what feels right. Bows will definitely be wary of an supernatural olympic-swimmer fly and are most likely to take a normally drifting and helpless critter.