Here's a quick post on how to cut your steerer tube yourself on your bike. Its not hard at all if you have basic tools and you are even modestly mechanically minded.
I recommend leaving your steerer tube long when you're setting up a new bike. This allows you to try different bar positions by swapping spacers around. It also makes it easier to sell your fork if you decide its not playing nice with your new frame. Here's a photo of my bike with an extra inch or so of steerer. I know... it looks odd. I'm used to being mocked so bring your best! When you're convinced that everything is dialed in its time to commit and size the steerer properly.
Next up, remove the headset cap and extra spacers:
Then, mark the steerer tube at the top of your stem. Press that sharpie in there hard to get to the corner and a nice straight line.
Now its time to remove your stem.
Now calm down... put down the power saw. Its not time to cut yet. You'll want 2-3mm of gap between the top of the stem and the top of the steerer tube. This is the part where you find out how much German heritage you have. If you get out your digital calipers you're more than 50% German (no matter what grandma says). Otherwise, you're just a slob like most of us. I scored a line with the cutter in the image below to give you an idea of what a proper cut looks like.
Make sure you check to see if your star nut is above or below the cut line. If its above press it down lower but don't go crazy. You don't want it too deep after you make your cut.
Ok, psycho its time... get to the cutting. Some people use a hacksaw with tape as a guide. I prefer using a pipe cutter because it gives a perfectly square line. You can also use the Park saw guide if you're going to be doing a lot of this. If you're using a pipe cutter be patient. Take several spins, tighten a little, several spins... you get the idea.
You'll hear a tink sound when the upper section separates. Its a glorious sound if you measured properly. If you didn't please contact my lawyer.
You can see I look like a seasoned pro with the star nut just in the right place. Would I lose any credibility if I admitted that I was lucky? Now its time to set the star nut to the proper depth. I recommend using a Park tool for this. Its cheap and it makes sure you don't press the star nut in crooked or damage the threads. Use a hammer and tap lightly. If you go nuts you run the risk of mucking up your headset.
Next you'll want to remove the slight burr that your pipe cutter left at the edge of the steerer tube. If you don't remove it you'll probably have trouble getting the stem and spacers on and off and it tips your buddies off that you do piggy work. File lightly and leave a nice chamfer all the way around. Keep the filings out of your headset. Bearings and metal shavings have a long standing feud so it never ends well when they hang out together.
Here's a shot of the steerer after being expertly filed. Its a little known secret that Moots used my filing as inspiration for their buttery smooth welds. Now you know.
The rest is easy! Make sure your fork is fully flush with the headset in the lower crown and put your stem back on. Don't tighten it yet. Add your stem cap and tighten it.
Next line up your bars with your tire and re-tighten your stem. If you have a Ritchey stem don't be ham fisted... there's a torque spec there for a reason.
Stand back and admire your work. Oh wait, I missed a step!!! Go post on the Weight Weenies forum that you saved 7/8 oz by re-sizing your steerer tube and wait to see if they figure out you're mocking them.