Like Rick said, don't worry about getting the insert straight. Once it's inserted into the blank, and the insert is screwed onto the stopper mandrel, the axis of the hole in the insert is the turning axis, and unless is misaligned or you are doing some kind of multi-axis thing, the turning will be symmetrical about that axis.
The problem that is solved by using a metal insert is that if the blank is wood, the grain usually is oriented top-to-bottom (ie, spindle orientation). That means that when you drill a hole in the bottom of the blank and thread it to receive the mandrel, the threads are perpendicular to grain and that makes them weaker than if they were parallel to grain. A metal insert solves this by making the threads stronger that the turning material itself.
The approach I take is sort of halfway between a simple threaded hole in the bottom of the blank, and a metal insert. I drill 5/16" holes in small scraps of wood - into face grain. Then, when I run a tap through those holes, the resulting threads are parallel to the grain of the insert wood. I then mount each scrap on my stopper mandrel and turn them down to form 5/8" cylinders that I can glue into a 5/8" hole in the bottom of the stopper blank. Once the glue cures, that leaves a 3/8" 16tpi threaded hole with the threads parallel to grain - stronger than a simple tapped hole in the blank, and much less expensive than a metal insert.
Metal inserts are fine, but my experience is that the type with a knurled outer surface (show in the picture that Bobby posted) are probably better than then more common type with a coarse male thread. The variety with a male thread are intended to go into face grain, and those threads just don't work well in end grain. I occasionally make canes, and sometimes I use a metal insert to allow a cane to be broken down into shorter lengths for travel. I tried standard male threaded inserts, but found that rather than screwing into end grain, they tend to split the grain. Knurled inserts combined with a two-part epoxy works better.
