I'll just toss my thoughts in the heap. I haven't tried them all, but I really like Tamiya plastic for stretching sprue, I have used it forever and get consistent results. I snip off a small length. I am not worried about corners or attaching points, as long as there is a straight portion in the the middle to heat. I hold the sprue by one end keeping a straight portion over the flame. I roll the sprue in my fingers over the flame so it melts evenly. I prefer to hold it further than closer to the flame I find allowing it to heat slower yields better results (usually 2-3 inches above). When it is heated enough the center will be shiny and the sprue will start to bend. I let it flop over so that the 2 ends are at a right angle. I lift the sprue away and grab the other end with my free hand and stretch. If you want a thicker length wait a second or 2 before slowly stretching the piece a couple inches. To make really thin stuff begin stretching as soon as you remove it from the flame. Stretch it quickly (not fast just quick) and evenly, as far as you can, but again how far you stretch determines how thick it will be. I can get strands as small as .002" by stretching as far as both my extended arms. I usually take a couple minutes every so often and stretch a bunch of sprue to various lengths and put them in a box or hang from pegboard. So I usually already have what I need on hand. Personal experiment and good plastic are the keys.

Age old top secret technique.

When stretching sprue: Stretch vertically, NOT horizontally ---- your sprue will remain straighter, and not develop any curve (sag) as it cools.

Another "nifty" trick....once you master the technique, try this...

using a sharp blade, change the cross section of the sprue ( rectangle,hex,oval etc. )
I use a scraping technique.... works fast.

Then, being careful to heat the sprue slowly, proceed to pull as described above.

If done carefully, you will end up with the same cross section.... only much smaller.

This is a great way to make very small bolt heads

I first learned this technique on glass - turning test tubes into pipettes. Amazingly, it works for that too! - although it's crucial that you hold the glass vertically.

Another tip from the same source: to cut sprue for this, don't muck about with razor saws or side-cutters. Simply nick the sprue with your knife and bend it; it will usually break at that point.