WhatÂ´s the lowest quality (in the compression percentage) do you recommend for audio and video?
80% is the general rule of thumb, but the size of your playback device, and the player you use for viewing is the govering factor. A good upconverting player helps with big screens.
Depends are you going to use single layer dvd media or DL media? But usually for best quality spend a little more and get 1:1 as the original for best quality. But also it depends on how much your willing to pay for the media and what you willing to sacrifice for play time? I for one don't mind the loss in quality when going from DL to SL media as the cost does come into play and plus for me I can live with SL media but other would decided differently.
There will be no compression with DL media. The question asked is, "what is the lowest recommended quality of compression", which implies the OP is using SL media.
I have gotten away with less than 50%, but it's really a bad idea to drop below the 70 - 80 % range.