Meh... it's just a badge. Do whatever you want, as long as you're not intentionally trying to make it seem like your car is more expensive than it is. I say that you can even use an ///M badge if your car is better than an M135i, as long as your badge reads something like "///M 135" or "135 ///M" or something else to make sure you're not trying to pawn it off as a 1M.
2008 135i Montego Blue sport pkg, premium pkg, Logic 7 sound, iDrive/Nav
Mods completed: JB4+ & FBO || LSD || BMW-P suspension + F/R sway bars || 18" Breyton GTS-R || M3 front bushings || Vogtland camber plates || BMW-P rotors + Stoptech pads |||| Acquired, not installed: BMS WW Meth kit || Logic HVI intake || Mishimoto OCC |||| Shopping for: Summer tires