I find that deters my speed, especially when playing on the upper strings (meaning low E, A and D). Putting your thumb back there helps with reach and it helps your fingers spread out more I find; cover more ground.

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you'll have better reach with your fingers behind, but its easier to bend and vibrato with your thumb on top. classical/shred/jazz players tend to keep their thumb behind the neck... while blues and rock players often keep their thumb on top... whatever fits your style really.
i notice to do different kinds of music i do that or dont like when i play korn and SOAD i put the finger behind the neck, and when i play some Jimi Hendrix or Bob Marley or somethin i put the thumb there. so depends on music
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I sort of switch back and forth between over the neck and behind the neck while I'm playing.

Over the neck helps with bends and behind the neck helps with finger reaching. Si I'm guessing whatever works for you.

allowing your thumb to creep over the top of the neck is bad technique when attmepting to play scales, or any single-note leads, as well as chords and such...however, when playing in a style where bends and vibrato are needed, the thumb over the neck is needed
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i heard that when you're playing, you're thumb is supposed to go behind the neck, but when i play, i rest my thumb on top of the neck, will this effect my playing?

Well, see how far you can reach compared to with your thumb behind.

This has all you need to know - http://www.guitarprinciples.com/Guitar_Technique/2nec_left_hand_pos.htm