I see, so folk in factories have magic cutting tools and mere mortals don't.....
I've shortened dozens of stems and never had a problem of any kind, although one thing that might catch you out is that the bolt often has rolled threads on it and can't be shortened with impunity; it varies a little but normally about an inch can be removed and you can still use the original bolt. If you need to remove more than this a shorter bolt is normally the best solution.
Then again I've made hollow stem bolts to replace the needlessly heavy solid ones too, so maybe I will take on jobs that others won't. Having said that, shortening a stem is probably the most straightforward job you could do...
Its for Roger to say of course but the problem he faces is a double whammy; not only do you have to sell the stem (at a loss presumably, since whoever buys it won't enjoy any warranty on the part) but you end up having to find another stem that wasn't originally your first choice.
cheers