I've always had to sand magpul parts. That goes for just about every part they make. Maybe I'm anal and don't like the molding seams, or maybe I'm old school and consider it the job of a gunsmith to use available tools to custom fit pieces together.
As much as the lego analogy gets passed around, they aren't legos. Plastic is much easier then metal to work on. Some sandpaper and a few needle files are all you need.