Unfortunately, that doesn't always work. Note the video of the city cop in Canton, Ohio who was caught verbally abusing a motorist who was trying to present his carry permit to the cop as required in Ohio and couldn't get a word in edgewise, then got another raft of grief for supposedly not informing the cop that he was carrying (while the city cop was on his initial tirade). With the downside being so great from running into the rotten apple in the barrel, the consensus on the various web sites I have seen is to not disclose carrying a firearm unless the state law requires it or you are being ordered out of the vehicle.