I read here something about this issue with stereo decoupling and how it is better quality to encode at -q 6 or above....

Newbie question is: What exactly is it and is it related to 'joint stereo' in mp3? Assuming they are related, then I don't see what the problem is as JS is pretty much a default in the mp3 arena. Assuming they aren't related, then I guess I have just learnt a new term

Also, is it a serious quality problem? I mean does encoding at -q 5 or -q 4 impart such a serious quality problem that I'll go crazy about it?

No, it's hardly noticeable unless you search for it specifically, and even then it's by far the better artifact to mp3's.