[This section added by John Allen]
Spoke nipple heads or access holes in the bottom of the rim must be covered with a rim tape, so they can't puncture the inner tube. Rim tape must be thick and strong enough to smooth out any sharp edges, and to withstand pressure from the inner tube if the rim has recessed or socketed spoke holes. Some rim tapes are simple hoops of rubber, similar to the rubber of the inner tube, with a hole for the valve. These rim tapes are adequate for use in wider rims without recessed spoke holes, but must be stretched over the rim when installed. Narrow rubber rim tapes often break at the valve hole. Better rim tape is an adhesive-backed strip, sold in a roll like handlebar tape, and has a hole near one end which fits over the valve hole of the rim. Placing a valve temporarily in the hole will prevent the tape from slipping during installation. The tape is laid down all the way around in the bottom of the rim, and the other end is trimmed just short of the valve hole. Duct tape or glass-fiber reinforced strapping tape can be torn lengthwise to the needed width and works too. Use three or four layers. If adhesive-backed tape can't be wide enough to get past the valve, it can just stop at either side.
The rim tape must be just wide enough to fit the bottom of the rim. Too narrow, and it doesn't cover what it needs to. Too wide, and it rides up on a bead seat of the rim as shown in the photo below, preventing the tire from seating correctly. The tire will sit too far out where it rides on the rim tape, and too far in at another place. It will give a bumpy ride, and there is a risk of a blowout.
Rim tape is not added until the wheel truing is complete.
I used to wrench in a shop for just a couple of years in '94-'95. I remember being taught to use strapping tape for rim strips, I think we pulled the strips from new bikes and sold them separately.....from what I remember in the bike shop business, all the money was in the bike accessories, bikes were the smallest profit margin.