There is a mould line across the top of most of the stormtroopers helmets (sort of runs shoulder to shoulder aligment).

File it off? Or is it part of the detailing?

Its hard to file off smooth without giving him a flat spot on the top of his head....

I used an exacto knife or hobby knife, I hear filing can leave the plastic rough on these minis.

I used the blade perpendicular to the mini shape to smooopth it off without cutting too deep.

The citadel mould line remover tool worked quite well for me

I use a blade to carefully scrape away mold lines. But with this type of plastic avoid using your standard hobby files - I use a nail file for the helmets in a very gentle, circular motion.

yup.. hobby knife. i tend to "scrape" very lightly away from me on the hidden mold lines. i can't really explain the scraping part.. just lightly drag the hobby exacto knife across the line repeatedly, don't cut too much you might gouge. after lots of minis you will no doubt find out what works best. good luck, and may the force be with you!