Carbon?
I'd have thought that would lead to fraying of the fibre.
Not unless you're a real heavy-handed muppet. I file the tops of carbon steerer tubes and they're fine. Just use a #2 rather than a bastard file.
You cut the steerer freehand and then file the wonky top flat?
That's what I cautioned against.
@Brommers started
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Carbon?
I'd have thought that would lead to fraying of the fibre.