I actually prefer a pipe cutting tool. They're about 3 bucks from the hardware store and make a nice even cut. They do mush out the metal a little bit, but that can be sanded and filed down easily after the cut is made, which you'd need to do anyways with a hacksaw.
Get a bi-metal blade; they are more expensive than regular blades but worth it. A bi-metal blade is basically hardened teeth bonded to a flexible/spring backing. The teeth are hard enough to cut most metals and not get dull, and the backing material is flexible enough not to shatter. A coarser (less TPI) blade will cut faster, and a medium to coarse blade is probably most appropriate for steerer tubes.
Rule of thumb for hacksaw blades is that the teeth should be fine enough to have at least 2 teeth in the material being cut. Coarser teeth cut faster. So for thin wall pipe you usually need a fine blade, probably 32 TPI.
I'll have to research ways to fit a bike stem, as I am only familiar with the setup that came pre-installed on my bike.
Any tips on fitting a stem before I cut too much off the steerer tube?
I like to leave a good 2" extra on the steerer tube and use a bunch of spacers for a while. This enables you to move the stem up and down, figuring out the height that feels right to you. Once you've settled on a good location, just take off the top cap and top spacers and make a mark on the tube. Cut about 1/8" below that mark and you'll be set.