No sights should tilt. IRL you would keep your sights horizontal to aim.
Quote from Hoak http://steamcommunity.com/app/222880/discussions/0/828937545907306854/
"On a related note (and very annoying to see in Insurgency), is that so far Ghost Recon and Call Of Duty 4 are the only games to get both weapon handling and camera perspective correct with respect to 'leaning', i.e. no tilting of of the camera and canting of the weapon sights when 'leaning'... Canting a firearm from any fire position is still strongly discouraged in most Mil firearms training (with the exception of some specialized CQC handgun paradigms like Castle's CAR Shooting System) and our brains keep then world upright regardless of our heads position..."
Hoak again at http://steamcommunity.com/app/222880/discussions/0/558747287962894992/
"Summarily, A canting camera and/or weapon is technically, metrically wrong in terms of proper weapon handling and perspective -- the world does not 'tilt' if we cant our body, head, or rifle; separately or in unison.
As well, canting an aimed firearm from any fire position is improper weapon handling, that is rigorously repudiated and corrected in virtually all firearms training -- military and civilian."