I am not a gun nut or anything, but I’m pretty sure those signs are useless. Not only do most people conceal carry, but any robber or shooter coming to your business to actually use a weapon is not going to be deterred by a no guns allowed sign.
It may stop one or two open carry dummies from using your business. Also, cops, obviously. That might be enough of a reason to put the sign up, still. I’d probably just put up a no cops allowed sign.