There's been posts about this in the past. MA requires one, but I refuse to put it on my car. I bought the mount for it though from JonB at partsrack and only use the mount on the front of my car when I'm getting my registration. Otherwise, I leave the plate in my trunk. I've only gotten pulled over for it once, but all I said was that when I park my car I take it off and put it in the trunk, because it can be easily stolen. Not that someone would actually steel a license plate, but I guess it could happen if someone saw the plate on an easily removable mount and wanted to be an a$$hole.
^The above excuse about being at a car show is a good one too, but I wouldn't want to deal with lying anymore then I'd have to in case the cop asked "what car show" I was at(more than likely they wouldn't ask that lol). Unless you said something like a friend of yours was having a get together with some cars taking pictures and you forgot to put it back on before you left.