I know several people that have "Caution K-9"on their vehicles. IMO, it is an attention getter.
I'd rather just put a DO NOT PET
collar on my dog if I didn't want people to approach. I know of a couple people that use the "K9 unit" velcro collar panel for their IPO dogs, I think that is misuse and shouldn't be done if they aren't affiliated with a dept.
There are plenty of shirts you can order with the word K9 on them, or hats.