Do you generally need to re-tap both sides if only one side is bad? - or is it just good practice?
With the older bottom brackets it is normally only the left hand one which deteriorates. This is usually because the left one is the adjustable cup and is held tight by the lock ring which can never -correction; rarely unless you have a suitable tool- be tightened as much or as well as the fixed cup on the other side.
There are other fixes available. Some frame makers avoided the problem altogether by not putting any threads in the bottom bracket. Those used a cartridge system. The cartridge was always an option to use in a bottom bracket with a damaged thread. Of cartridge systems there are at least three different types using different implementations.
If you need to know more, just PM me. I do not think this public forum is a place for cycle engineering. Anyhow, I cannot be bothered with the technology to take the photos and upload.
At last the iPhone 5 has been released so things may change for me.