For commercial vehicles I think it's a good thing to have signs because they are such an easy target for chancers. People know that a commercial vehicle, especially and HGV will most likely be fully insured, the company and contact info will be displayed on the vehicle usually so they will try an off-the-books settlement first and depending on the vehicle they could damage another car without suffering any damage themselves - so easy to put a fraudulent claim in. We get them all the time including the common "something fell out of the back of the truck and hit my car". If they think it's got cameras then I'd assume they'd think twice about putting a claim in.
The truck of ours which has a four camera setup has already paid for itself after an owner of a brand new XC90 tried to claim we had reversed into his car. The cameras proved nothing happened and suddenly there was no further contact from them. Unfortuantely, the front facing only cameras trucks probably wouldn't have helped in this instance.
As for private cars, I don't want anyone knowing there's cameras installed for three reasons:
1. I don't want to create a situation where they try and take the camera after an incident and I'd end up with an assault charge by stopping them.
2. I don't want the police to take the cameras and then "lose" it and there goes my proof of what really happened.
3. From past experience the majority of people are quiet honest at the scene and then suddenly become lying b*sta*rds when it comes to the insurance claim. I want these people to be prosecuted for fraud so if they don't know about the cameras then there's a better chance they will lie and can be caught out. Our insurance brokers and underwriters are very aggressive with going after people who submit fraudulent claims.