Just to be clear on this, the top cap will fit "perfectly" as long as no part of the steerer is sticking up beyond a line a couple mm below the top of the stem. The critical thing is that when you tighten down the top cap, that it bears completely on the top of the stem and doesn't touch any part of the steerer whatsoever.

Beyond this, its mostly cosmetics. If you hack it off at an angle or roughly, it will still work fine unless you're a real gorilla and do it so badly that you actually cut it down far enough that you start reducing the area for the stem to clamp properly.

That being said, a nice/straight cut is the mark of a professional job.

- Mark