Regardless if you put something up correctly or do something wrong or miss a small detail. Accidents happen. Insurance is a necessary evil. However, as for the OP's question..... all signs are made and installed with the mindset, they are being done in a manner conforming with the standards of many codes from licenses, to building, to electrical, to construction. If negligence can be proved, then you run a chance, your insurance company will not represent you. When something goes wrong in today's world, it seems all the lower class lawyers jump on board with their 99 finger pointing technique, where anyone from the guy making the extrusions, to the painter, to the people building the sign, to the codes department, to the inspectors, to the landlord's approval or not. Everyone gets blamed. If you throw enough sh!t at it.... something's gottta stick somewhere. That's their MO.
Yes, we have some paperwork needing to be signed according to what type of sign it is, where it's going and whatnot. However, putting disclaimers and waivers in it doesn't do you any good should something eventually go wrong. Now, I said eventually. There is a time table that will take place. Nothing is expected to last forever, but whether you are working on an electric sign or a hanging sign, a pylon or whatever, if you see problems, they should be reported immediately and either the end-user pays you to fix them or gets someone else to fix them. If you just go on doing your work and ignore fixing a problem, then it will become your responsibility, since you were in there and did nothing about it. Should something go wrong and you say you didn't see it or notice it, you are admitting you don't know your line of work, therefore should not have been doing it in the first place and could be found 100% guilty of negligence and fraud.
There is a common train of thought of how business gets done in every line of work from estimating to the actual work to finishing it up. Stay within those guidelines and you should be fine. Start dancing around, doing things you aren't capable of, but doing anyway, be ready for the wrath of the system.
If you really want a good answer on this... ask a lawyer..... and see how much he wants to charge you to make some run-on sentences in your quotes and invoices will cost ya.......