I didn't trim and it fits just fine. I know where you are talking about and just like derek said, it still clips and contacts metal. I guess you could dremel it but I wouldn't tear up the bed rails, especially if it works without it.
2011 Lariat Screw 5.0
Boss 302 | aFe CAI | SCT X3 from 5Star | 2" Rough Country Level | 20" Fuel Throttles | Borla Pro XS | N-Fab Light Bar | PIAA 520s & 540s | Craven Speed Shorty Antenna | Interior LEDs | HD Heads & Tails | USB2 | Extang Tonneau | Plast-dipped mirror reflector, rear wheel wells, and grill