Hmmmm I'm not so sure. I'm in the UK not in the states, so the laws are different I'm sure, but in UK you only need a licenced electrician when adding/removing an electrical circuit and for any electrical work in the bathrooms. So say adding a new spur from your fuse box to add electricity to your garage or a garden shed - that sort of thing. Any work on existing circuits(adding sockets, light switches, new light mounts etc) is fair play and doesn't require a permission or any paperwork. Instead you just get an electrical inspection done and an electrician signs off on the house as a whole.

Like someone else said - when you buy a house you don't know how the electrical wiring was done and by whom. Paperwork is required only for the things I mentioned above - new circuits added after the house was built mostly. And even then someone would need to prove that it was done after construction.

So even if the insurance company wanted to deny a claim due to a DIY fitted socket....I think they would struggle, as there's absolutely nothing forbidding me from installing one, and most importantly - my contract with the insurance company doesn't forbid me from doing so.