If it's not a M car, it shouldn't have a M on where the model designation is. You can do whatever you like, but you just get ridiculed by BMW enthusiast, ie, you just look silly.
Now, having M badges on body parts are generally okay as they signify M parts, like the M pack etc, or the M license plate is very acceptable.
Having a M on the other side of that balance of the 135i is probably okay, and I can accept the M fan argument.
The biggest no no is having the ///M where the 135i is.
Just like this loser:
http://www.1addicts.com/forums/showthread.php?t=269616