Can I cut the steerer tube with a pipe cutter?

A pipe cutter is an excellent tool for this application. Properly used it results in a perfect, burr free cut that is 90° to the longitudinal axis of the steer tube. The outer aspect of the cut remains burr-free by virtue of the rollers smoothing the outer aspect of the workpiece as they pass over. The tool, like any other, should be in good working condition with a nice sharp cutting wheel that is not deformed or dull. The rollers opposite the cut wheel should be smooth and free of foreign material that could scratch the workpiece or cause the tool's cut wheel to not track true. Size matters as well since pipe cutters are rated for different ranges of pipe diameter. Thus, a pipe cutter that includes 1⅛" diameter in it's working range should be selected.

Here is a link to a PDF file outlining the proper use of a pipe cutter. A few things I'll add to this tutorial is that in order for the outside of the cut to remain burr free on completion of the cut, the rollers should follow the leading cutter wheel. It really doesn't matter which way you turn the tool for a proper cut, however. Clockwise rotation with one's right hand aids the biomechanics of having to turn the adjuster clockwise every rotation or two. Keep in mind that it's a process of scoring the metal with consecutive passes of the cut wheel and there shouldn't be excessive clamping force applied to the steer tube by over tightening the cutter's adjusting wheel.

Using a pipe cutter is not recommended for cutting a carbon fiber steer tube. A fine blade hacksaw, a saw guide, and respiratory protection from carbon fiber dust generated by the cutting process are called for in this situation.