I rewind until I feel the tug of the film coming off the take-up spool and stop there. I keep meaning to stop just before it comes of the spool, because I think it would easier to get the film out of the camera (M2) that way. I find the curl in the leader can make removing the film a bit fiddly. Some cameras put a crease in the end of the leader, so I always put two creases in to indicate an exposed roll. (I almost never rewind a part-used roll, but it seems like a sensible way to ensure there is never any chance of confusion.) I also put in two creases because I've found that the spool on my IIIf doesn't grip some films very well - they can slip out during loading, so I put in a single crease to improve the spool's grip.
I have never seen any ill effect from having only the half-width part of the leader protruding. Light-piping aside, film seems to do fine whether the full width is sticking out, half the width, or the film is fully wound into the cassette.
I also think the scratching risk when loading into a tank is something of a myth. If it really was a risk that one needs to account for, then the net, forums and photo books of yore would be full of tales of people scratching films because they used half a roll, wound it back, and reloaded it later to finish it off. Running the film through the velvet light trap doesn't scratch it. Running it through a trap with grit in it scratches it. Film comes in little round cannisters for a couple of reasons, one of which is keeping it free from foreign material. If you want to minimise the risk of scratching, minimise the time the film spends outside the cannister and outside the camera. No more than a handful of seconds.
When loading onto a reel, I trim the end sqaure and snip off the corners in the light. Then I put it in the changing bag, and load straight from the cassette. Handling a roll of loose film is more fiddly than drawing a few inches out of the cassette, giving the reel a few twiddles, and repeating until the last time the film is drawn out results in it stopping. Aside from that, loading straight from the cassette gives you the option of winding the film back into the cassette if there is a problem with loading. Once you've popped your crimped cassette open, you're committed. If you start having a problem, you risk sweating, and because you now have 5 feet of loose film lying around in the bag, you risk getting sweaty paw prints just where you don't want them (and that will make the hands more sweaty).
Caveat: I don't take pictures in sandstorms. If I did, I might alter my method.
