I generally use a metal hose clamp (a "jubilee clip") of the requisite size for cutting steerers. Once you tighten it up medium tight, it automatically positions itself perpendicularly to the steerer axis, and provides a good cutting guide. Remember, the old maxim "measure twice, cut once" is particularly relevant when cutting steerers - mistakes here can be pretty costly. Not sure which pipe cutter you're talking about, but I presume not one that is used to cut copper pipe - these rely on the relative softness of the copper, and likely won't get through a steel steerer