I would think signatures are needed for any type of insurance work, they do that to set the limits of the expectations of the job, like using restoration materials, or salvaged parts to repair their vehicle, so the next problem that comes up is covered. That is really how you are protecting yourself. I looked into the software for auto repair software, you basically put your hourly rate in and it spits out an estimate by an established time frame for that type of repair to get made. Your rate would include your overhead and your own hourly wage. They figured out how long it should take a shop to make a repair and bills accordingly. It might help you to see if you have been charging enough along the way, but getting a signature ahead of time is just protecting yourself for that person that comes back complaining about fit or floppy metal. It does not sound like you have had many customers that complained that you did not do what you said you were going to do. I also think paperwork is important for your insurance, if something catastrophic happened while you had possession of the car.
As far as being a previous customer of body shops, man, the last thing I needed to hear was about the shops overhead, I just want my car back like it was before the accident or before the floors fell out, its your choice to buy bigger and better equipment that will allow you to work faster at higher profit rates. Bottom line, what would you expect from someone you dropped your personal vehicle off at? What if they decided to make a bunch of repairs you did not ask for and billed you for it? That is usually the best way to decide, put yourself in the customers shoes and if what you are doing is enough, adding software and a system is basically adding cost to your overhead. It will allow you to estimate faster and show them that any shop will expect to spend the same time on the repairs.
Just seems like its a step to make yourself feel more like a business and not just side work, but with that, comes all the pains of feeling like you are going to get screwed every job.