I was wondering, do I have to use my thumb for fretting a note on top string? Is it okay if I don't have to? I found this site about guitar lessons and they use their thumb for fingerpicking.
Yeah you can use the thumb of your fretting hand to fret a note on the top string, it isn't uncommon. Whatever, if it feels right and comfortable, then do it.
yeah, Hendrix used to do it alot. I do it every now and then although when it comes to barre chords or 5th chords then you should get into the habbit of using your 1st finger to barre on the root note and your other fingers to play the 3rd, 5th etc.

Also when fingerpicking, the sort of correct technique is to use your thumb for the bass notes i.e E, A, D and asigning your 1st finger to the G string, 2nd to the B string and 3rd finger to the high E string, but I don't always do this but it's seen as the correct way.
Dave Tither
i think its a bad habit personally and if you rely on it it could become a problem, using it every now and then if fine though
Whatever works, I couldn't do it for a while, but recently got it down.

A good song with usuage of thumb fretting would be 'Who Did You Think I Was' by John Mayer.
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It's a bad habit if you can only play like that, "correct" postition is with your thumb in the middle of the back of the neck, but it's a bitch to bend notes that way, but also with your thumb up top, try playing the C major scale, it's almost impossible, so it's good to be able to switch. . .
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I will chime in with the majority. You should ONLY use the thumb to fret the low E
string in very specific situations. If you are covering up your lack of ability to
fret with a finger, that's bad.

Fretting with the thumb can free up a finger to do some additional fretting on
some chords.

Thumb behind the neck and curled around the neck are both valid positions
depending on what you're doing.