I hang a sign when maintenance comes through, only because I hang one anyway warning them to watch the doors because my cat seems to think it's funny to slip out. At the bottom I put "also harmless live reptile"When the pest control comes through I hang a sign warning them where I store the food for the cat and ask them to also keep away from the room Quetzal's in. They've assured me that their spray only kills bugs, but I'd prefer to not take chances.
Signs can be a bit of a double-edged sword. The "beware of dog" thing I've only heard of once, and it was a "did you know" discussion between friends and I'm not convinced it's not an urban legend. However, a sign does alert people to what you have in your possession and may cause trouble for you. One of my neighbors saw my cat sign and complained to the landlord (original lease = no pets), and if I hadn't cleared my animals with them prior to moving in that could have been really bad for me. On the other hand, the signs give me a little piece of mind when their intended viewers watch out for the things I'm warning them about (such as my cat making a mad dash to the outside world) and I feel confident that I can return home to find both of them safe and sound.
I personally like your sign idea. When I lived at home and had to call 911 (long story, went through a window), the medical team read our Beware of Dog signs and were aware that they were coming into a home that housed at least one (three) large dog. That's a lot better than opening the door to three unexpected big German Shepherds that didn't bark when you rolled into the driveway. Your sign would be doing the same; letting them know what to expect so they don't panic or have a kneejerk reaction when they get surprised.