Hey Matt!!! That's a very good question and is a problem I had to figure out through trial and error. The video I learned from did not mention it in detail.
The trick to getting a good taper on your dread tip is the way you hold the backer rod over the heat source. Holding the rod directly over the heat in a vertical direction will allow the heat to glide up the rod and shrink it more smoothly. When the rod starts to taper, take it from the heat and roll the tip of the rod against a flat surface to sharpen the point. You will need to find that sweet spot between the rod and heat source where you will not burn the foam. This will also straighten out the curve the rod gets from the packaging. Use some scrap rod to practice with first..... It took me a couple of tries before I got the hang of it. Hope this helps!
[]
[]
[] < backer rod
[]
[]
[]
[]
/\\\//
\\\\///\\ < heat source
#######