A momentary lapse of conscience whilst soldering could lead to catastrophic surface damage on your instrument or the customers instrument.
But a quick and easy solution is often the best. Make a template that will surround the cavity where you are working on the guitar - it's usually on the back side of the instrument and this area is usually covered with a plastic guard. I use the pick guard as a template. If you are doing soldering in other area's you will need to become creative and take measurements in order to replicate a negative image of the surface you are working with.
In the photo, I used simple card board to prevent the soldering iron from burning the guitar if I happen to get sloppy or I am unusually distracted. I hold the cardboard on with duct tape, gaffers tape or blue masking tape. Regardless of what tape you apply, pre-test (in an innocuous place) it to be certain you can removed when the project is completed.