The vehicle has to be repaired to an acceptable & satisfactory standard to put it back to the condition it was in immediately prior to the accident. You have to sign a form to say as much otherwise the repairer doesn't get paid, & he could also be held accountable if the repair isn't roadworthy. Don't expect the insurance company to pay for more than is necessary though, eg. don't expect a full re-spray if you've only scratched the front bumper. You can of course pay for any extra work yourself done at the same time. I believe that you are quite within your rights to demand that your vehicle is repaired by someone that you are happy with & not by the company the insurance want it repaired by.