Starting with this as the basis for the screw body, I would simply bridge the outer-most edges to create one solid tube, then chamfer the spiral edge to "extrude" the threads from the shaft. Then there would be some work to build the top of that shape into the head of the screw, and the bottom into a point, all while trying to maintain as many quads as possible. Using this technique, however, I seem to always end up with several triangular faces. I haven't found a strategic way to avoid that when it comes to building the screw from a helix ribbon. This technique would also take a little longer; probably 15 to 20 min to get a decent looking screw or bolt  (I suppose I could never claim to be very good at speed-modeling. ;)

Yes. thats the way I went about it. Spiral (360deg) and edge extrude until the two ends met, then cloned. I think the tris are fine there, as you say they won't be seen.