If you want high quality start at quality 4 (~128kbps) and go up if you can hear a difference*.
If you want to be (relatively**) safe use quality 6 (above that even the guys with the 'golden ears' don't hear much improvement).
*To be sure you really hear (and don't just imagine) a difference you can make an ABX test, rip a track you think sounds different to .ogg and uncompressed .wav (or flac, or rip to wav and make a compressed version with eg. oggdrop), then either use foobar2000's built-in ABX comparator or decode the .ogg version to wav using eg. Winamp's diskwriter and compare both versions with eg. ABC/HR
**no lossy codec is perfect